it was in the tutorial. Go to the starter home and there are two garages. the one on the right is your warehouse to store 10 items.

The left one is the vehicle garage. On foot click on the door and you should see the car swap ui appear. switch then load it back up to sell your old one.

You dont need to go to the starter "house" - every house that you purchased works. All of them have a garage.

Originally posted by biomekanic:

I upgraded to the truck, and it says "New vehicles appear in your garage." I go home, nothing. Go to the pawn shop, nothing. Where is this garage?

Those roller door looking things? How are you not seeing them...

OMG.... 2nd quest of this game SAYS "INTERACT WITH GARAGE TO TAKE OUT CAR"