*cracks knuckles*
Right, here we go:
First off, as the tall one says, different apartments offer different levels of heists. These are usually best done with mates as randoms in this game are generally completely devoid of any mental activity at all. When playing solo, I find doing the normal missions great fun if frustrating at times. These are found by going to the letters on the map, the first set are from Lamar and are highlighted on the map with a yellow Ld. The first will involve trying to start a war between two gangs by dressing up as the other gang and killing everyone. This take two teams of two and can be a bitch if you have idiots in matchmaking. That said, I find that the further I get in the story, the higher the players are and the more skill they seem to get so stick with it. These missions are usually great fun and rewarding, and are one offs, as opposed to the ones that have various stages which you trigger in your apartment. Definitely keep those for mates if you don't want to claw your eyeballs out. I find doing these open world missions to be the most rewarding as there's an actual story behind them and you can watch the game evolve as you go. Definitely my mode of choice at the moment. Heists, for the record, don't seem to follow any story, and nd are just random group activities.
Secondly, if you're a bit bored and don't have much time, hold down the touchpad and select "daily objectives". These are three things you can do each day to get XP and cash. They are all sorts of different things from losing a four star wanted level to taking a flying lesson, to finishing a round in a shooting range, to whatever. These are different every day and also different from player to player, so if you do yours, they'll be different to mine. Sometimes these are quick, sometimes they take ages (losing a four star wanted level took me about two hours to pull off, but now I know the tricks, it'll be much quicker next time). Also when roaming the world, you will occasionally find red circles on the map. These are random events and usually involve killing a load of gangsters. They're fairly easy, and will net you about £7.5k. There are loads more thing to do in the open world but I'll get to that eventually.
The first thing you should do if you have a bit of cash is buy an apartment. This will be your base where you can store your cars and start heists. Based on the level of apartment, you can unlock different heists. I only have a high end apartment at this point so only have access to the high end stuff but from some reading the other tiers of apartments offer different tiers of heists. Once you have an apartment, it's time to find a ride. I went for something which is reasonably fast, can drive over almost anything and can take a battering. It's bright pink & purple and is nicknamed the Fagwagon. I'll post some pics up later. You can nick rides and keep them but be aware tha the high end stuff needs to be bought, you can't steal hypercars and store them as the game says they're too hot. Once you have a ride, go to Los Santos customs and stick a tracker and insurance on it. This means that if it gets nicked / destroyed, you can ring up the insurance company and they'll replace it at their yard, which will appear on the map. This is done by pressing up, going to contacts, and somewhere near the bottoms is Mors Mutual Insurance. Ring them up, wait for the menu in the top left of the screen to appear, and select your car. Then grab whatever wheels you can get your hands on and go get it. Whilst at Los Santos customs, also stick whatever else you want on it, the amount of customization in this game is ridiculous.
Once you have a ride, it's time to tool up. Go to an arms shop (little gun on the minimap) and buy your guns. Also buy some armour, buy the highest level you can and as many as you can (any spares you can carry will go into your inventory for later use, keep reading). You'll need the automatic pistol, an SMG, sniper when you unlock it and a homing rocket launcher (this will be your bread and butter until you're a bit more familiar with the game, it's basically a big FUCK OFF button to anyone harassing you).
Once you've got a flat, a ride and some lead, it's time to venture out. As mentioned above you can do almost anything in this game, but the first things that will happen to you are many deaths. If someone keeps killing you over and over, you can ring lester and put a bounty on their head. Simply press up on the D pad, contacts, lester, wait for menu in top left, place bounty, select name, select amount, press X. Boom, you've now put a hit on someone. Later on in the game you can also hire a mugger to rob people, I've had this happen to me a few times, and this guy is relentless. He dies easily but has very handy situations. I got yanked out of a tank by one and he nicked the fucking thing!
I'll leave it at that for now, but @GazzaGarratt, @Stretch616, you lot know what to do now. I can't be the only one tearing in to @Greboth all the time.
Any questions just ask.