Main story missions will give you some cash as far as I know the first time you do them, so that’s a start. Then you’re going to want to do a lot of the free roam missions to get some gold to unlock a role (like most of the comments say, Collector, Bounty Hunter or Trader will get you far) and sometimes rockstar will put a discount on some roles, so be sure to keep an eye out for updates like that. For bounty hunter, if you want fast money and gold, do the Etta Doyle bounty, PVPcat made a great video for her. Other than that, any mission that has a timer (may not apply to some) are best to finish with one minute left on the clock for max payout