I am active duty enlisted. I came in open mechanical and got lucky and got my first choice of jobs, jet engine maintenance. Everyone is right, make sure you come in with a guarenteed AFSC.
Things to consider, the job you get will determine the length of your tech school. Also, once you get out of tech school and get to your first base you will be given your CDC's, Career Development Course. They are additional written study material that will teach you more about your job. Ususally several volumes in length. You will have a month to complete each volume and be tested at the end of each volume, and then there is an End of Course, EOC, test that covers all the volumes once all your CDC's are completed. Ok, with that being said, you will be doing your hands on training, studying your CDC's on your down time and your supervision won't let you take college classes until your CDC's and EOC are done. That's about the first year of your enlistment.
Once you have your upgrade training done you can start taking classes. It is 100% tuition assistance so all you have to pay for is the books. I'm finishing up my BA thru Embry Riddle and also working on my flight training at the same time, not to mention working full time typically 11 plus hours a day. The big thing that I would try to impress upon you if you do enlist is, once you have the chance to start taking classes do it. Don't wait. Good luck.
KC135guy