Cash on GTA Online is a fine delicacy- If you want to make use of the best features in the game without spending real money on cash cards, there are certain things you can do to squeeze more money out of missions.
Most Money from Holding up a Store
The higher your rank, the more opportunities there will be to get a lot of money in GTA Online, but at a low rank, holding up a store is probably the best way to make money.
However, instead of forcing the cashier to hand you the money, shoot him in the head, and take the money from the till yourself by pressing right on the D-pad at the till. You will usually get more money for doing this and you don't have to risk getting shot by the shop owner.
From here you can open up your phone, deposit money into your bank through the maze bank website and step outside and get killed by the cops. Because you are still a low level, you'll only lose a portion of what you just stole on hospital bills. You can do this to all twenty stores in San Andreas and it is probably the safest and quickest way to get money in GTA Online at a low level.
Mission at Rank 15
At rank 15 and above, a mission called Violent Duct. This mission can take any time to complete, but it is usually done within a minute. You'll get $9,000 for doing this mission, and with a dedicated crew, you could be making $100's of thousands an hour. This is definitely one of the quickest ways to get money in GTA Online right now.
This is the easiest and safest ways to get money. You can sell cars to Los Santos customs as long as they aren't too fancy, and you will get 10% of the cars worth. Check online to see the cost of the car you are driving, and then take it to Los Santos without scratching it to get the best reward. This can only be done every 48 minutes but can reward you with up to $10k.