After killing an enemy, press Circle and 50 Cent will kick the person, and most of the time cash icons will appear. Additionally, do all the missions as far as you can. Then, go to the subway entrance and choose a previous mission that you have completed to get more money in that mission.
After completing the game, go to Iceberm and buy the "Invulnerability" and "Infinite Ammo" cheats. Then, go back to the subway entrance and choose the Boat mission. Repeatedly complete that mission. This is the mission where you must get the drugs, making you a lot of money. Use that money to buy more cheats, music, clothes, and different ammunition.