Run around customize cars, do missions, Basically anything you want to do

Well to begin with I'd probably say do a lot of missions or races with your friends cause you need to be a certain level to unlock different guns and to get missions that give you good money and RP like Rooftop Rumble you need to be level 75+ but if you have a friend that level they can host it for you and you will be able to earn a lot of RP and Money that way.

JUST HAVE FUN!
There really are no aims in the game, kill people, run them over, kamikaze some planes into people, annoy people then get into some sort of epic car chase, just have fun!:

Basic stuff,
Do jobs for money/xp
Stay away from most people because they will more than likely kill you repetitively
Do races if you like car customization
Its easy once you know what you're doing..

Just play different missions, you'll get calls off some of the singleplayer characters resulting in a job most of the time (Lester, Martin, Simeon, Lamar, Gerald, Ron, Trevor) but yeah if you just want money asap just get a friend to host Rooftop Rumble (a mission unlocked at level 75.) Other than that just do what you'd do in the singleplayer pretty much
Also just a tip about player blips, redder the blip = the more dangerous they are, Whiter the blip = the more friendly and passive they are

Doing car races will get you chrome and levels up on ur cae