Rayman,
Take a look at the following link:
http://www.staynavy.navy.mil/enlisted/content/Commissioning_Paths/default.asp
There's no reason you cannot apply early on for STA-21!! It's very competitive... I know when I was picked up for BOOST (1996), which is now part of the STA-21, being warfare qual'd., good evals, volunteerism, etc. was important.
Good luck!
Robin