You can steal and sell a car about every 45 min to an hour at LS Customs. SUV's will usually net you the highest gain. Contact missions can also bring in a good amount of cash, especially on Hard, or if you do them on a bonus day. You should be able to average 25-30k an hour like that. There are also money glitches that can be used....but Rockstar is pretty good about being on top of those, so as Scrublord stated, make sure they are recent. At a low level, a Survival mission is a good way to get weapons without spending too much money, as even if you don't make it all the way through, you still retain weapons and ammo you picked up till you log out. Once you get about 200k you can get yourself a Luxury Apt with a 10 car garage and the room that allows you to launch your own heists. I suggest saving cash until you get the amount needed for the Apt or a 10 car garage first. the lower end apts are pretty useless.