You want good advice? -Don't join the service!
Now that that's out of the way, here's what is pertinent if you choose to ignore that valuable advice:
I served >9 years in the Army. Basic training was alot easier than I expected (Ft. Relaxin Jackson) and AIT was just a familiarization course with the
computers, and manuals. Oh, and alot of d&c, and getting yelled at. Nothing you can't handle if you go in with the right mindset.
If you have some college, join straight as an officer.
Even if you don't, you can go 'Highschool to Flightschool' and become a Warrant Officer. You still go to regular basic, but at the end of it you
get promoted to SGT, and go straight to Flightschool as your AIT. Or you can get in as an enlisted, but apply to the 'Green-to-Gold' program, and go
OCS.
Jobs requiring a sec-clearance would be along the lines of MI, PAC, Commo, etc. Unless you're in the right job from the start (unlikely), you have
better chances of going into the 'secret-squirrel' world as an officer.
Don't let your recruiter talk you into anything you don't want to! Sign nothing! Get a bunch of suggestions from him, and heavily research them.
There's no such thing as a lost opportunity when it comes to recruiting. They always need soldiers in all the fields. (that's for when they want to
pressure you with a 'limited time offer')
So much for the Army. I've heard the Airforce is a cakewalk, and what I know of them sure points in that direction (PT-test based on heartrate...

), however the Coast-Guard seems to really have a sham-job.
Nothing against the Coast-Guard, now! They excell at fulfilling their part of the service! It just is a fact that there's a difference in bearing
when you compare, say, the Nasty-Guard, with the Reserves, and the Active-Duty guys... get me?
So, anyways -Don't do it, man!