Well you can get in a taxi, copcar, firetruck, ambulance, and do there side missions by pressing R3 while your in the car, and that can score some big bucks, and some rewards to.
Or you could go to another mission if there is another one to choose from, it should be located on your map.
Or you could kill people and take there money.
Or just look up the other side missions and tips on this site. Because I dont have the time to list them all.