The Asvab is no big deal. If you're really concerned, pick up a study book and prepare. Most don't and do just fine. Although, in the Coast Guard at least, the better you did, the more schools you qualified for. And if you're not too commited to the Air Force as of yet, check out the Coast Guard. Best branch of service by far...although I'm partial as I just retired after 20 years... Yesterday as a matter of fact...I already miss it...

Here's a forum where you can learn about the different ratings (jobs) and general info about the CG...
http://forums.military.com/eve/ubb.x/a/cfrm/f/67919558

Joining the CG was the best thing I ever did...they'll give you every opportunity to go as far and accomplish as much as you want.