honestly just go cam for now... at 18, with an agressive cam, headers, LS6 intake and a good tune, you're going to make WAAAAAAAAAAAAAAAAAY more power than you know what to do with or can handle.....
Everyone is saying heads and cam which are great ideas, but I want to make sure that you know that when you go with that combo, you MUST get headers at the same time or you wont get anywhere close to the full potential out of it.
If you're going that route or mod'ing your car period, DO IT YOURSELF... I can't stress this enough... if you just throw your money or your parents money at it and don't work on it, you won't respect it much as well as you won't know how the hell anything was done and you'll just be another squirel... (SQUIRRRRRRRRRRRRRLEYYYYY!)