Go to the Ganton Gym in Los Santos. Go to the bicycle and start at level 1. Work your way up to level 10. After awhile it will say that you have worked to your limit and to return. Keep pedaling and your stamina will start to increase. Use the following trick to get a lot of weapons, increase your muscle, increase your stamina, and get a "Hitman" rating with any weapon. After the initial mission, go to the Verona beach area (on the south part of the map near the ocean). There is a free open gym here. Use it to increase your muscle and stamina. when you need to eat, there is a free hot dog stand down the path east of the gym, or just save the game at your house. To get "Hitman" status with any gun, find the weapon of your choice and shoot people or cars. Try going to any parking lot and shooting cars. It is easier and they respawn if you ride a short distance away and return. You can get as much money as desired doing the fire truck missions, then going to the off track betting after getting at least $10,000. Bet 10% of your money on the last horse until you win. If you lose too much money, reload your game and return. Repeat this process until you have a good amount of money.
