You're getting great advice. I'll second what Yuki said about being clear on training. Don't go by the "title" of the MOS either; they always sound sexier than they really are - Supply Logistics mean you work in a sweltering hot or freezing cold warehouse moving boxes.
Also consider that if you pick a field to go into, that field may never be stationed where you want to go. My first four years there were 2 places in NC, CA and Okinawa each for me. Once I re-enlisted there were a few more but mostly in the same States.
Let's say you pick the Navy and want to be a Corpsman. Chances are you'll end up where the Navy has hospitals or clinics (land and sea) unless you aren't lucky and instead you get sent to a Marine unit (Marines focus on the other end of the cycle of life so they use the Navy for medical) that will take you with them where ever and when ever they go.
When picking a field, consider the training. I went into electronics. Great field right (was in the 70's & 80's). The first two weeks was 8 hours a day of math! Then the boring stuff came. This went on for over a year! Being stationed for school at 29 Palms CA didn't help matters. Middle of the frickin' Mohave Desert. It isn't like high school or college. Once you start a course, you stay on topic until you are done. I took an electronics course my senior year. The course lasted my entire senior year. The Marine Corps completed the same course in just over a week.
If you flunk out, you don't get out. Now you have not held up your end of the bargain so they pick a new field for you. Something that won't be as challenging. Most of the guys that funked out of radio repair (where I was) went to radio operator. So now, instead of fixing radios in a climate controlled maintenance shelter, they got to carry the radio (25# plus spare batteries) along with all their other gear. The rest went to artillery. That was my motivation to pass.
If you can narrow down the field you really really want, find someone that was RECENTLY in it and ask them what their life is/was really like. Shouldn't be hard to find someone nowadays. Usually you get to pick the field and the Service will pick the exact job - YMMV. So going into the computer field and you could end up fixing computers, working on networks installing LAN lines or something similar. I've been out for 12 years so it may be different now but most all the services worked things fairly similar and tended to use the same schools. I've been to Air Force schools though I was in the Marines (great duty by the way - being a Marine on an Air Force base rocks!).