races
missions
cruising with players = 200+ xp
getting away from cops
jobs
if you go to stats on the start menu go to awards and look at an award you want to complete you get the xp shown on the award (easy way)
kill other people gives you money and if they don't have money you get xp
capturing bountys

You can bounty boost if you don't care about your KD, Or if you want to level legit then just do missions when you get them

Quickest waywould be to find 2 others willing to put bounty on each other and letting you kill em. Other then that doing missions till lv 20 untill you can put bounties on someone and swap kills.

I find robbing stores is a good way to make a little dough.

Also selling cars is LSC can yield some nice results are long as the cars are in good condition (can repair before selling, but takes from your profit)

Potshot it's a mission unlocked at level 25 and pays 1450rp and $7000 each time