We have a whole guide for how to make money in GTA Online, but that also includes activities that are only available to higher-level players. So get in the habit of periodically dropping by an ATM (use the interaction menu to find the nearest one) and depositing your cash. If other players kill you, you'll drop a portion of the money you have on you. If you suspect people aren't playing fair, you can jump to a different session from the main menu.
This makes you invulnerable to other players but also prevents you from using weapons or attacking, so it's best used when you're peacefully exploring and don't want to be interrupted.