Qualifications
If you're considering becoming a military trauma surgeon, you'll need quite a bit of training and qualifications before you see your first action. After high school, you'll be required to earn a four-year degree from a college or university in a field which prepares you for medical school, such as biology or anatomy, or one of the many specialized pre-med programs. After that comes four years of medical school, for which you'll have to pass the standardized MCAT to get in.
You have a couple of options at this point (source). You may choose to attend a private medical school (which you would pay for) and then apply to one of the branches of the armed forces (who will help pay off medical school loans). Another option is to apply to the military's medical training school (which is paid for by the government) and immediately enter the field upon graduation. Either option requires a commitment to years of military service, so being certain of your path is essential.