Look, kid, the best advice I can give you is don't focus on the now. Focus on the later. As hard as it may be for you to hear this, you're not going to get the game. At least not now. So take your time to play every other game in existence. I assume your parents will let you play pretty much any other game, right? I'm willing to bet money that you haven't played about 90% of the good games that are out there. I don't know what systems you have, but you obviously have a PS2. There are stores out there that sell primo PS1 games for $10 a piece. I'm not talking Spongebob, I'm talking Resident Evil, Syphon Filter, Gran Turismo, Crash Bandicoot, Metal Gear Solid, the Final Fantasy series, games that are destined to be classics. You can build an impressive collection like that. Forget about the game, wait a while, and pretty soon your parents will allow you to play it. Just don't expect anything now.
If you're still in denial, you can try to access the game in other ways. Play it at a friend's house. I bought a lot of my games after I played them at a friend's house first. Although this probably won't help at all, discuss (don't argue) some of their misconceptions about the game. Tell them that the game is only fantasy violence and doesn't lead to violent acts. Hell, even if you were a psychotic killer, playing this game would keep you busy and let you unleash your murderous rage on the game rather than the real world. Tell them this game and video games in general are popular because they have the same concept as that of a roller coaster: all of the thrill, none of the consequences.
If that doesn't work and you still want to experience the thrill that is Grand Theft Auto, play the original game. I believe GTA, GTA2, and GTA: London now come in a bundle pack you can get for $30. The violence is not nearly as real and the graphics suck, so you might, just might slip this one by your parents. Despite having sub-Super Nintendo graphics, these games are actually so big that the PS1 couldn't handle them. All the elements of Grand Theft Auto are there, the cops, the gangs, the missions, the carjackings, the cities so big you get lost in them. And the original games, with the exception of London, have three levels in them. They also have the pain-in-the-ass top-down view and are a lot harder to beat than their 3-D counterparts, but they are the makings of the greatest video game series I've ever played.
Good luck.