How do I become 18D if i already signed to 68W?

My mos is 68W health care specialist, but i want to be 18D Special Operations Medical Sergeant. I leave for boot camp june 20 2011. Is there a tryout at my AIT in Fort Sam Houston, or should i have singed up from the beginning? I really want to be either a combat medic or 18D. I got a 90 on my ASVAB and im not in the best shape yet. Right now i can only run a 13 minute two mile if that helps.