Whenever you buy a car, the vehicle will be added to whichever garage is set to your default. To change the default garage, open your menu, go to settings, and switch between the garages with the arrow keys. Your 5 options for garages are in Paleto, Vespucci Garage, Legion Square, Commercial Lot and Sandy Shores. When at a garage open your menu and select retrieve vehicle. Once your vehicle is out, you can drive it away. When you want to return the vehicle, head back to any of the garages and select return vehicle in the menu.