Join the Reserve Officers Training Corps, or ROTC, while attending school. In addition to taking other academic classes, you also take ROTC courses that prepare you for military service. The education is not free, however: you are required to complete a stint in active duty military or the National Guard.
Talk to a recruiter before you enlist in any branch of the military. When you enlist, you can opt in to invest in the GI Bill. There are two types of bills that pay for the college tuition of former service members: the Montgomery GI Bill and the Post-9/11 GI Bill. With both types of GI bills, you set aside a certain percentage of your military pay for college, which the military matches. You can use the benefits to pay for some or most of your college tuition after completing your term of service.
Enlist in the National Guard, also known as the reserves. As with active duty military, you can also take advantage of the GI Bill if you join the National Guard. Unlike the benefit for active duty enlistment, which is used after you've completed your military service, National Guard members can be called up for service in the middle of their education because of their reserve status.