Heres some really good tips if you need money fast.
(not in order of most effective)
WHAT YOU SHOULD DO:
Probz the easiest way to get money. The first couple of missions don't pay very well (about $500), however *SPOILER (SMALL ONE)* as you start working for Vlad your missions start giving you 4 figures.
2) LJ's diversion's:
If your relationship reaches 60% with Little Jacob then you unlock the ability to do work for Little Jacob which involves driving a shit box car to a destination to complete a drug deal. I personally loves these missions and they are a great way to make quick money at the start of the game. Each delivery gives you $500 for a literately 3 minute trip.
3) Blowing up Safe Vans:
I have forgotten the name of these vans (sozz) but you cant miss them. They are big Grey steal vans used to transport money from banks etc. If you come across one steal it and then blow it up. About $5000 is dropped when it is blown up
4) Stevie's and Bruises Car Missions:
Earlier in the Game you meet a guy called Brucie (one of romans friends). After a few of his missions he will start to send you pictures of cars that he wants you to steal for him (via the internet). If you go and collect the cars and bring them back to his lock up (not damaged) you will get no less then $1500.
I'm not up to the point in the game where you unlock Stevie so i have no idea of what to do with his missions.
WHAT NOT TO DO: