You can only save the game in your Hideout. If you look at the mini-map display in the lower left corner, the location of your current Hideout is represented by a purple house icon. When you go there and park your car in the garage, walk into the door next to the garage, and you'll be able to save the game. When you return, six hours of game time will have passed, and your car will be repaired. Be advised that if you are currently in a mission, you will not be able to save your game.