What collection type should I use?

Busineses use applications such as Order Entry and Inventory and Accounts Receivables. Within AR there are customers. I think your application should be designed in a similar manner.
 
So the cars would be the inventory. Your orders (whatever you want to call them) will consiste of customers and order items. Your application is more of a services-oriented application and there are many applications that have been developed for that. You might be able to find samples that can help you.
 
One possibility is to use the DataTable class for the collection. Doing that will be more work for some things but it will be less work over-all. You can use the DataTable class even if you are not storing the data in a database, but I assume you need to store the data somehow. Whatever you use for storing the data, the DataTable class will probably make it easier.

Hi
I would like to do it with car objects, but where do save the information about the rented car. I mean who rented it, when it was rented, how long is it rented, cost etc.
I could add it all to the car object, but it would be a messy object, when was it rented doesn't seem to belong to a car. From OOP it wouldn't be good, of course I am new to this field, so I don't know where to make compromises between OOP and efficency. Or maybe I should make a new loan class?
Also what's with Google Chrome not workinging when I try to reply, it displays the loading but it doesn't complete, I can't type in text into the field.

ArrayList is really a bad idea. Go for a Collection class like
Class Car
{
 public string CarModel {get; set;}
 public string CarOwner {get; set;} 
}
Class Cars: ICollection
{
 List oCarList;
 public Cars()
 {
  oCarList = new List }
 public void Add(Car item)
 {
  oCarList.Add(item);
 }
 public void Clear()
 {
  oCarList.Clear();
 }
 public bool Contains(Car item)
 {
  //Implement as you need
 }
 public void CopyTo(string[] array, int arrayIndex)
 {
  //implement as u need
 }
 public int Count
 {
  get { return oCarList.Count; }
 }
 public bool IsReadOnly
 {
  get { return false;}
 }
 public bool Remove(string item)
 {
  //implement as u need
 }

 public IEnumerator GetEnumerator()
 {
  foreach(Car car in oCarList)
  {
   yield return car;
  }
 }
 System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
 {
  foreach(Car car in oCarList)
  {
   yield return car;
  }
 }
}
 
Now you can create a Car object & add it to cars collection. Simple right?

Hi Moose.
You Can use a 2D array of Objects and with new operator change them in specific class object.
For Example:

object[,] MyArray = new object[n, 4];    // n is e.g number of cars
for (int i = 0; i < n; i++)
   MyArray[i, 0] = new Owner();             // Your Defined Class
for (int i = 0; i < n; i++)
   MyArray[i, 1] = new Car();                  // Your Defined Class

And so on.

A boolean will do just fine. The other option you have if you want to get granular is an enumerator

Hi

How could I make a list with array objects, so I could insert many things into one list position.
List Cars = new List(); ???

Well I guess it is a simple library system, I have 4 classes for now: Owner/Client, Car"Library", Car, CarSpec.

I have thought about the structure of this thing.
So, Felipe, you think I should have a availability boolean in Car class ?

ArrayList class is one of the most flexible with many methods and properties. However, it does not support multi-dimension.