There are several different options. First if you have already taken college courses prior to enlisting in the military you may be eligible for the college loan repayment program. What service you enlist in determines the amount you will receive. The Army pays more while the Air Force pays less. It will not pay all of your loans back but it helps.
If you want to go to college while in the military you do so on your own time. The military will pay a percentage of tuition and fees associated with a course. You are responsible for payment of your books. I know the Air Force pays 100% of tuition and fees. Not sure what the other services pay.
