This could be a bit of a long answer.
There are many legal ways to make money such as apple/peach picking or diamond/salt mining (to name a few). To pick you just need a car/truck and need to go to a picking field marked on the map and press the left windows key to pick. To mine you need to buy a pickaxe at the market and go to a mining area and press Y and then use the pickaxe.
There are also illegal methods such as cocaine, heroin, weed and meth. These do give a higher return but you are likely to be stopped by police or rebels which can result in you loosing all of your drugs and your truck.
These legal and illegal goods need to be processed before they can be sold (with some expectations such as frogs legs and peaches).
There are professional jobs such as taxi driver, bounty hunter and ARAC (Car repair man). These however require a lot of money to purchase a licence (1-2mil).
There are also government jobs such as the police force or the NHS medical team. These will require an application being sent through the forums and proof of maturity and experience (in the server).
I would personally start with some sort of mining job such as copper, diamond or salt and then move on to drugs if you feel up for it and don't mind the prospect of losing everything. Then I would move to the NHS or police as after this time you are likely to be used to the server and all it has to offer.
Hope this helped!
Frisky
Edit: This oughta help