It's a long application process. You can look the Interservice Physician Assistant Program online and see the requirements. I would say that unless you currently have a scholarship or are in some other way uniquely invested in your current schooling, I would recommend joining now, then doing more schooling on the military's tuition program. I think what helped me get into PA school was the experience I had in conjunction with the fact that I earned all my credits while being deployed and was successful in both aspects.

I went through the Interservice Physician Assistant program. You earn a BS degree after the first 16 months (Phase 1), then you earn a Masters Degree in Physician Assistant Studies in the following 12 months (Phase 2).