Note: Some of these hints may only work on the unpatched version of the game.
Create an invite only game session, go to any store, and rob it. Punch the clerk so they all run off, then take all the money in the cash register. After that, enter the Start menu and go to Xbox Live Marketplace as if you were going to buy in-game money. Once Xbox Live Marketplace loads, exit it and you will spawn back in the same store at the exact same spot, except the world will have refreshed. You can then attack the clerk again and take the money from the cash register. Repeat this process as many times as desired. You can get about $100K an hour using this glitch.
To make easy money at the start of GTA Online without alerting the police, steal a good common car in Vinewood. Once done, go to a Los Santos Customs car shop. You can sell stolen cars to car shops, although most car shops will not accept luxury sports carst. You will get about $9,000 to $10,000 for a good common car that does not have significant damage. After earning some money, deposit it at an ATM to avoid potentially losing it to other players or if you die. Once you are satisfied, buy some body armor and weapon customization to do heists and robberies to earn more money. It is recommended you buy a silencer for your weapon when you have enough money so you can avoid heat from the cops.
When you reach Level 45, the "Mixed Up With Coke" mission will be unlocked. The earnings for completing this mission are $18,000 and 3,500 RP. It can take more than 15 minutes to complete the mission the first few times, but once you figure it out, it can be completed with three other players in less than two minutes. Note: One player will need to be at Level 45 to access the mission, as well as one player at Level 19 or higher and two players at Level 9 or higher to do the mission.
When you reach Level 75, the "Rooftop Rumble" mission will be unlocked from a guy named Martin Madrazo (leader of a Mexican narcotic gang in the Los Santos area). To complete the mission, you just have to kill five people. It is possible to earn up to $2 million and 400K RP per hour by repeatedly completing the mission. Note: You can also do the mission if you are at a lower level and get invited by someone at Level 75 or higher. If you are at a lower level, make sure to use sticky bombs instead of the grenade launcher, and kill any survivors with a rifle.
The "Rooftop Rumble" mission will be unlocked when you get to Rank 75, from a guy named Martin Madrazo (he leads a Mexican narcotic gang in the Los Santos area). To complete the mission, you must kill five people. It is possible to earn up to $2 million and 400K RP per hour by continuing to complete the mission. Note: You can also do the mission if you are at a lower rank and get invited by someone at Rank 75 or higher. If you are at a lower rank, make sure to use sticky bombs instead of the grenade launcher, and kill any survivors with a rifle.