In most Grand Theft Auto games, you can save cars by driving into a garage by the player's home. This would automatically make the car available again just by driving it out of the garage. This is a bit different in GTA IV's case, however. Some homes or safe houses in the game have garages that you can park your car in to save it. Meanwhile, there are other in-game houses that don't have garages. In this instance, there's usually a defined, reserved parking space at the front of the building. Just find a parking spot with yellow lines around it, and park your car within this space to save it. This usually appears on the side of the road. Once your car is parked in this space, it'll appear there again when you return.