If you don't want to do heists, you can do Snipers vs. Stunters with your friends. I think it's way more fun, can get you some good laughs, and with a lot of people can get pretty hectic!

I grinded the -blam!- outta Rooftop Rumble before R* nerfed the mission payouts

Started a new topic: >Be me

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.

Money glitches found on YouTube make sure they are 3 or less days old or they won't work

Tip #1 kill everybody in public lobbies, because they WILL kill you

To put it simply. Just do stuff. Freemode events. Contact missions. Selling cars. Survivals. Then once you are past 50 or so do heists. U unlock heists at lvl 12 but nobody wants to play with a low rank who will most likely die.