First, you fill out the FAFSA to procure federal loans, which bring in around 18K a year. Then you contact your school's financial aid office once you get in to take out private/bank loans, your school's loans, or state loans, etc. For this, you would likely need your credit reports (the school itself would tell you how to do this, if you're curious now, just email their finaid office).
Regarding military HPSP, I've thought about that also. To do that, it's a little late in the game now, but still possible to apply. You would have to contact your area recruiter to get the information on applying (this also requires a physical, etc.). Expect to fill out a lot of paperwork! Google HPSP to find their 800 number - those people can then get you in touch with a local recruiter.
HPSP basically works like this. You get a 4 year scholarship. You then do a residency at a military hospital and after that, you are required to serve at a military base, hospital, or wherever they send you for 4 years (+2-3 more on the reserve list, I think). During med school, you do OBC your first summer and then do some rotations at military facilities. If you want more exact details, go to the "military residencies" forum of SDN - the people there know a lot more about than I do. The general advice they give is that you should do it more out of a dedication to the military, than simply for money...but I tell you, the financial benefits are awesome!