The extreme lack of money in GTA Online (without buying CashCards) is a hazard almost everyone encountered at least once in their career as an infamous gangster. There are ways to make money easily, though, so once you know what they are, you can really improve your gameplay.
Find an appropriate vehicle.
Since Rockstar has restricted the selling of vehicles, you’ll have to find one that meets the following requirements:
The car has to cost under $50.000, if stolen from street/driving NPCs.
If you want to sell a bought vehicle, it has to come from legendarymotorsport.net, southernsanandreassuperautos.com, or warstock-cache-and-carry.com, as long as the vehicle can be stored in a garage. Also, keep in mind that you’ll always get only half the amount of money you spent on the vehicle. For example, if you buy a car with buy price $2,000,000 and you spend another $200,000 on modifying it, your sell price will be $1,100,000.
You cannot sell vehicles obtained from Warstock Cache and Carry if they require Pegasus for delivery, or ones obtained through glitches or exploits.
Open the Interaction Menu by holding down the SELECT button (PS3), the touchpad (PS4), the VIEW button (XBox) or M (PC).
Select Mod Shop (also known as Los Santos Customs) and confirm the selection.
Make your way over to the waypoint and drive into the garage.
If your vehicle is damaged, there are no options shown to modify it.
Select Sell Vehicle and confirm.
Completing Missions or Jobs
Start a mission/job.
Make your way over to a blue mission/job circle and start the mission.
Complete it to get JP, RP and – most importantly – money.
Find a store.
Open the map and scroll down to the entry store and create a waypoint. Make your way over there.
Enter the building without having your weapon out. Go to the counter and draw a (semi-auto) pistol (not the AP pistol for example).
Intimidate the cashier by shooting around him. If a single bullet hits him, he will run away from the register into the back room and come back out with a pistol – he will kill you if you don’t kill him first! Anyways, as soon as the cashier emptied the register, take out a shotgun and kill the cashier.
Get out of the store, get the nearest car and travel to a Safehouse – being wanted on level 2 is no fun.
Stealing from Pedestrians
Kill anyone on the street and collect the money.
Keep in mind this method is not recommended.
The police are called almost instantly and – most of the time – you won’t get any more than $50.