Building on what Airborne Eagle said,
You could join the USMC Platoon Leaders Class. This is what I plan on doing.
This is the info I got from Marines.com
"This program allows students to complete the requirements for Marines commission without interrupting your academic career. For college freshman and sophomores, PLC is two six-week training sessions at the Marine Corps Officer Candidate School in Quantico, Virginia. Juniors attend a 10-week session. PLC training occurs during the summer months.
Travel costs, meals, textbooks, uniforms and lodging are provided by the Marine Corps, and students are paid for their time. Additional financial assistance may be obtained for participation in active duty. Upon graduation with a grade average of C or higher, students are commissioned Second Lieutenants of Marines.
The benefits to becoming an officer while still in college are numerous. You'll receive valuable leadership training during the summer that does not interfere with school-year activities. You obtain confidence and experience in the most committed team environment possible.
You may receive tax-free financial assistance of up to $7,000 for Platoon Leaders Class participation. In addition, you will earn up to $2,985 during your training. Most colleges will grant academic credit for your summer training."
You can get more info about PLC from these sites
www.marines.com/officer_programs/platoonleadersclass.asp
www.marineocs.com/portal/index.php
www.marineofficercandidate.com/plc.htm
Semper Fi
Good Luck