The eastiest thing to do is to find a "Armored Truck", these are normally around banks and High End Retail districts. You want to steal it and put it in a Rim Jobs or your Crib garage. Go to the far northwest Rim Jobs, get the truck and drive to the gas station pump. Get out and shoot the gas tank and get the money from the car that was blown up. Return back to Rim Jobs and repair the car. This is the easiest way to get money fast, just keep repeating this as many times as you want.