Here is a short list with five items explaining how to make money in GTA V / GTA 5.
1: Be a taxi driver!
Steal a taxi, then you can accept taxi jobs around in the game, if yo drive nicely, you will even get tips for the rides. You can earn upwards of $300 for a single ride.
2: Kill people and steal their money
You can rob people after killing them, the problem with that is that the police will get on your tail. Not very viable in the long-term… PS: If you hang around an ATM, then rob them those who withdraw cash, you are more likely to earn some more cash.
3: Go on the stock market
You can invest money in the stock market, it is not a guaranteed profit, so you should not do this unless you have money to lose.
4: Rob armored cars
If you see a blue dot on the map (unless when the cops are after you!), that means that there is an armored car there. If the car is parked and you see a person walking towards it with a suitcase, you can earn $5K by shooting him. You can also steal the car and break open the back doors.
5: Rob stores
Any store that is open for business, can be robbed! By walking into a store, then slowly going back, so that your character keeps the doors open (with the doors pointing outwards), you can equip a weapon. Shoot the cash register and they will open up and you will get your money!