Have you taken the ASVAB yet?
If not I highly highly highly suggest you study your butt off for it. A few points means a world of difference in what job you'll be placed in.
I was in the Marines, so while my experience is a bit different, it's still very simular. All I can say is the military is a big chance for you to really succeed in life if you play your cards correctly. If you do 4 years or 40 years, you're going to have life experiences that can not be given in the civilian world. You'll do things and see things that you'll have stories as well as friendships that will last a lifetime.
Like others said, you'll go farther the more you keep your mouth shut and ears open. And for heavens sake, iron your damn uniform! Us Marines love to make fun of the "nasty army" for having unpressed cammies and nasty looking boots.