Need some Help with Multiple Clinets - TCP Server

I need some help. I've been searching the internet for days, but i dont  get it together. I have a code for a TCP Server. It runs just fine, the only thing what is lacking is the possibility to send a message to one specific client (like a Pivat Message).

How do i put Multiple Clients into this code??

I'm a total Beginner, so I hope someone can help me.
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using System;
using System.Collections.Generic;
using System.Collections;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.Net;
using System.Net.Sockets;

namespace Server
{
    //The commands for interaction between the server and the client
    enum Command
    {
        Login,      //Log into the server
        Logout,     //Logout of the server
        Message,    //Send a text message to all the chat clients
        List,       //Get a list of users in the chat room from the server
        Null        //No command
    }

    public partial class SGSserverForm : Form
    {
        //The ClientInfo structure holds the required information about every
        //client connected to the server
        struct ClientInfo
        {
            public Socket socket;   //Socket of the client
            public string strName;  //Name by which the user logged into the chat room
        }

        //The collection of all clients logged into the room (an array of type ClientInfo)
        ArrayList clientList;

        //The main socket on which the server listens to the clients
        Socket serverSocket;
      

        byte[] byteData = new byte[1024];

        public SGSserverForm()
        {
            clientList = new ArrayList();
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            CheckForIllegalCrossThreadCalls = false;
            try
            {
                //We are using TCP sockets
                serverSocket = new Socket(AddressFamily.InterNetwork,
                                          SocketType.Stream,
                                          ProtocolType.Tcp);
               
               
                //Assign the any IP of the machine and listen on port number 1000
                IPEndPoint ipEndPoint = new IPEndPoint(IPAddress.Any, 1000);

                //Bind and listen on the given address
                serverSocket.Bind(ipEndPoint);
               
                serverSocket.Listen(4);
               

                //Accept the incoming clients
                serverSocket.BeginAccept(new AsyncCallback(OnAccept), null);
                ;
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message, "TCPServer",                    
                    
            }           
        }

        private void OnAccept(IAsyncResult ar)
        {
            try
            {
                Socket clientSocket = serverSocket.EndAccept(ar);
               
                //Start listening for more clients
                serverSocket.BeginAccept(new AsyncCallback(OnAccept), null);
               
                //Once the client connects then start receiving the commands from her
                clientSocket.BeginReceive(byteData, 0, byteData.Length, SocketFlags.None,
                    new AsyncCallback(OnReceive), clientSocket);
              
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message, "TCPServer",
                    MessageBoxButtons.OK, MessageBoxIcon.Error);
            }
        }

        private void OnReceive(IAsyncResult ar)
        {
            try
            {
                Socket clientSocket = (Socket)ar.AsyncState;
                clientSocket.EndReceive(ar);
               
                //Transform the array of bytes received from the user into an
                //intelligent form of object Data
                Data msgReceived = new Data(byteData);

                //We will send this object in response the users request
                Data msgToSend = new Data();

                byte [] message;
               
                //If the message is to login, logout, or simple text message
                //then when send to others the type of the message remains the same
                msgToSend.cmdCommand = msgReceived.cmdCommand;
                msgToSend.strName = msgReceived.strName;
               
               
                switch (msgReceived.cmdCommand)
                {
                    case Command.Login:
                       
                        //When a user logs in to the server then we add her to our
                        //list of clients

                        ClientInfo clientInfo = new ClientInfo();
                        clientInfo.socket = clientSocket;     
                        clientInfo.strName = msgReceived.strName;
                        button1.Text = clientSocket.GetType().ToString();
                        clientList.Add(clientInfo);
                       
                        //Set the text of the message that we will broadcast to all users
                        msgToSend.strMessage = "<<<" + msgReceived.strName + " has joined the room>>>";  
                        break;

                    case Command.Logout:                   
                       
                        //When a user wants to log out of the server then we search for her
                        //in the list of clients and close the corresponding connection

                        int nIndex = 0;
                        foreach (ClientInfo client in clientList)
                        {
                            if (client.socket == clientSocket)
                            {
                                clientList.RemoveAt(nIndex);
                                break;
                            }
                            ++nIndex;
                        }
                       
                        clientSocket.Close();
                       
                        msgToSend.strMessage = "<<<" + msgReceived.strName + " has left the room>>>";
                        break;

                    case Command.Message:

                        //Set the text of the message that we will broadcast to all users
                        msgToSend.strMessage = msgReceived.strName + ": " + msgReceived.strMessage;
                       
                        break;

                    case Command.List:

                        //Send the names of all users in the chat room to the new user
                        msgToSend.cmdCommand = Command.List;
                        msgToSend.strName = null;
                        msgToSend.strMessage = null;

                        //Collect the names of the user in the chat room
                        foreach (ClientInfo client in clientList)
                        {
                            //To keep things simple we use asterisk as the marker to separate the user names
                            msgToSend.strMessage += client.strName + "*";  
                        }

                        message = msgToSend.ToByte();

                        //Send the name of the users in the chat room
                        clientSocket.BeginSend(message, 0, message.Length, SocketFlags.None,
                                new AsyncCallback(OnSend), clientSocket);                       
                        break;
                       
                }

                if (msgToSend.cmdCommand != Command.List)   //List messages are not broadcasted
                {
                  
                    message = msgToSend.ToByte();

                    foreach (ClientInfo clientInfo in clientList)
                    {
                        if (clientInfo.socket != clientSocket ||
                            msgToSend.cmdCommand != Command.Login)
                        {
                            //Send the message to all users
                            clientInfo.socket.BeginSend(message, 0, message.Length, SocketFlags.None,
                                new AsyncCallback(OnSend), clientInfo.socket);                           
                        }
                    txtLog.Text += msgReceived.strMessage + "\r\n";
                    }

                    //txtLog.Text += msgToSend.strMessage + "\r\n";
                   
                    //txtLog.Text = msgToSend.strMessage;
                }

                //If the user is logging out then we need not listen from her
                if (msgReceived.cmdCommand != Command.Logout)
                {
                    //Start listening to the message send by the user
                    clientSocket.BeginReceive(byteData, 0, byteData.Length, SocketFlags.None, new AsyncCallback(OnReceive), clientSocket);
                }
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message, "TCPServer", MessageBoxButtons.OK, MessageBoxIcon.Error);
            }
        }

        public void OnSend(IAsyncResult ar)
        {
            try
            {
                Socket client = (Socket)ar.AsyncState;
                client.EndSend(ar);               
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message, "TCPServer", MessageBoxButtons.OK, MessageBoxIcon.Error);
            }
        }

        private void button1_Click(object sender, EventArgs e)
        {
            // Sendbutton
        }
    }

    //The data structure by which the server and the client interact with
    //each other
    class Data
    {
        //Default constructor
        public Data()
        {
            this.cmdCommand = Command.Null;
            this.strMessage = null;
            this.strName = null;
        }

        //Converts the bytes into an object of type Data
        public Data(byte[] data)
        {
            //The first four bytes are for the Command
            this.cmdCommand = (Command)BitConverter.ToInt32(data, 0);

            //The next four store the length of the name
            int nameLen = BitConverter.ToInt32(data, 4);

            //The next four store the length of the message
            int msgLen = BitConverter.ToInt32(data, 8);

            //This check makes sure that strName has been passed in the array of bytes
            if (nameLen > 0)
                this.strName = Encoding.UTF8.GetString(data, 12, nameLen);
            else
                this.strName = null;

            //This checks for a null message field
            if (msgLen > 0)
                this.strMessage = Encoding.UTF8.GetString(data, 12 + nameLen, msgLen);
            else
                this.strMessage = null;
        }

        //Converts the Data structure into an array of bytes
        public byte[] ToByte()
        {
            List result = new List();

            //First four are for the Command
            result.AddRange(BitConverter.GetBytes((int)cmdCommand));

            //Add the length of the name
            if (strName != null)
                result.AddRange(BitConverter.GetBytes(strName.Length));
            else
                result.AddRange(BitConverter.GetBytes(0));

            //Length of the message
            if (strMessage != null)
                result.AddRange(BitConverter.GetBytes(strMessage.Length));
            else
                result.AddRange(BitConverter.GetBytes(0));

            //Add the name
            if (strName != null)
                result.AddRange(Encoding.UTF8.GetBytes(strName));

            //And, lastly we add the message text to our array of bytes
            if (strMessage != null)
                result.AddRange(Encoding.UTF8.GetBytes(strMessage));

            return result.ToArray();
        }

        public string strName;      //Name by which the client logs into the room
        public string strMessage;   //Message text
        public Command cmdCommand;  //Command type (login, logout, send message, etcetera)
    }
}