Security in winforms c#

Question

private void btnCustomFind_Click(object sender, EventArgs e)
        {
            con.Open();
            SqlCommand cmd = con.CreateCommand();
            cmd.CommandType = CommandType.Text;

            cmd.CommandText = "select Contact from Login where Username='" + txtName.Text + "' ";

            txtNumber.Text = Convert.ToString(cmd.ExecuteScalar());

           con.Close();

           return;

       }

 

So above is my code when the user enters their username, their contact will show.

However, due to security reasons, if the user knows 1 or more different usernames, the user can easily gather their personal particulars. I have a database called "Login" where i keep every User's login. 

1) How do I limit the Find to only one user? The one user being the one who logged in.