How to make a truck

I wanted to know how to make a trailer attach to a truck during the game. I wanted to just make the cab, then when you pull up to a trailer, you can attach it to the cab. I modeled a truck cab w/ the trailer but the whole thing turned like it was one piece. I want the trailer to do two things.

1) I want it to attach to the cab during the game.
2) I want the trailer to turn separately frome the cab just like a real truck.

Please help me with this guys.

Thanks