All USAF pilot candidates complete phase II (T-37/T-34/T-6) and select their follow on track near the end of that phase (based on performance). The tracks available are T-38, T-44, T-1, and Helos. The number of slots for each can change each class. Students who select helos then go to Ft Rucker, and Im not sure where after that.
Years ago you would be selected as a Helo canditate prior to UPT, this isnt the case anymore. No one knows which track they will follow until near the end of Phase II.