It starts off when your dad (in the game) dies and you have to deliver this sword to your uncle, named Kenny, but after you get off the plane, you get jumped by these people and they almost kill you, then throw you with thier car at sea. After you get on land (and get used to the strange control scheme), you go into your safehouse(you can buy more of these as you progress through the game) and start doing missions for kenny(like stealing cars/delivering drugs etc). Soon after, you start helping other people... thats about it. Just like the other GTA games, there are many different weapons, you get a gun at the beginning but you can get more weapons as you kill specific people(kill police for beating stick etc.).There are also many cars that you can steal, and you can go to Pay and spray stores to change your car's color(to get away from the cops or just for no reason).
Bottomline, this is a great game, but if you think you wont like it, rent it first, but i recommend buying it, its good enough