Starting off in GTA online: What should I do?
1: Play the starting missions. All those messages you get in-game are there for a reason. The quick, easy missions available to any player help get you start the game, allowing you to begin your slow climb in the leader board. These little missions don't give you enough money to buy the fastest car in the game, but you will get enough to at least buy yourself a decent gun to get started on another mission.
2: Very self explanatory. Never go to hacking. Although the easiest way to get to the top, it ruins the game for everyone else, and will get you banned quick.
4: Take advantage of the double XP modes. Although they are not always that fun, you should take advantage of the double XP adversary modes. When stunt races first came out, I spent a solid hour playing the same, shortest race over and over again which eventually benefited he by giving me enough money to buy a new car. My best piece of advice: find the easiest, shortest adversary double XP game, and play it over and over, which is one of the easiest ways of getting money in game.
6: Lastly, know your priorities. You don't need to buy that golden gun camo, just save up to get yourself a high end apartment to start heists. The hardest part of starting GTA V for me was saving my cash, and not senselessly buying everything I can afford.
Hopefully this guide helped you make something of yourself in GTA V online. These are the steps which helped me the most, and I hope they do the same to you.