At some point of the game, I think it's after your first mission with Brucie, he will start sending e-mails asking if you can steal some cars*.
Some of them are pretty easy to steal but a couple have armed guys around that will start shooting at you, both are motorcycles so be careful. Also another couple have policemen around, the first one is a car that is being inspected by the police so at the moment you steal it you will gain 2 wanted stars and the other one is a bike in front of two police men so they will start shooting.
You will earn more money if the car is in good conditions.
*When you reply accepting the mission you will need to do it at that moment.