I'm also turning 18 in August. I see many kids my age who want to get into guns because of video games and movies. I also see many kids who have a legitimate interest but outside of these media nobody has ever taken the time to educate them. Plus, what is America if we can't enjoy the decision to indiscriminately buy an AK when we turn of legal age? haha
For the reference, dboy, as the mods around here are fond of saying, we must be ambassadors. This means being educated and well-spoken. Simply buying a rifle because it's got a "stronger round" and fires from an assault weapon type rifle may not present a good image to anti-gunners. Instead, figure out what you want. Do you want a range toy? An accurate AK that won't jam in any caliber should do that. Do you want something intimidating to whip out if somebody breaks into your house? If so, a shotgun would be a cheaper option, but if an assault weapon type is what you want you will ultimately buy one regardless.
As far as picking a rifle: Go to a gun store and ask to handle a few rifles. Tell them you are about to turn 18 and want to get a feel for different rifles so you know what to save up for. Get their FFL info while there so you can have the rifle shipped to them when you want to buy (this will probably make them more amenable). Try The AK-47 and 74 if they have it. Also try different AR-15's. Learn about your different ammo types. Ask them for their experiences. 5.56 mm is standard for an AR-15. Light recoil and easy to manage. AKs use 7.62 X39mm ammo or 5.45 X 39mm ammo. A 7.62 will kick more and could cause you to have bad shooting habits. There are also other rounds like .308 (7.62X51mm) which is used in hunting and "sniper rifles". It has a lot of kick but can shoot over 1000 yards in the right hand. There's Remington 6.8, 6.5, even a .50 Beowulf if that's what you want to get into.
Why don't you let us know what you're planning to use it for and maybe we can help a little more.