First, you take the ASVAB test to see where you'd be best in. My particular test showed I could take on any job (as per their scores), but my areas of expertise was anywhere mechanically- and electrically-minded.
From there, your scores would actually narrow your field of search down to a few areas which will help you further find a better job for you. Don't just take the recruiter's "job-choice" as he's paid to try to get you into a field that is currently in high-demand, though often those have sign-on bonuses and re-enlistment bonuses.