Also make sure you're getting physically fit as well (or if you're a parent, that your son or daughter is - though it doesn't hurt for parents either
).
What I did was I went through the online application with my guidance counselor the first week of school and submitted it on September 11, 2009 (I chose that day for personal reasons to officially start my way to being an Air Force officer). Then went to my gym teacher, told her what I was doing and she started me on a good plan to build up to taking the PFA in mid-October (I was already in good shape, though some "specialized" training doesn't hurt).
All forms were submitted by the middle of November, which was also the same time I sent in college apps (goal was before Thanksgiving). Had my interview on December 2nd, but my package wasn't completed in time for the first board, so I just got my answer on Monday (waiting that long is a real pain).
I probably could've had the whole process done a little sooner, but there were intervals that I had planned out to get things done, and it looks like it worked.
My advice is start the process in May like aglages said, because that gives your more time to complete all of that and any other ROTC scholarship and academy applications. Also, in May you're still in school, so if you want to show guidance your online app before sending it in, that's a good time to do it.
Do not start after September - it gets hectic by then, and depending on where you live, it may be uncomfortably cold by the time you do the PFA (particularly for the run since most school tracks are outside).
You want to hit this with a running start and grab it by the throat. I waited until September for my own reasons, but I strongly suggest learning as much as you can now, and starting in May/June. Also, ask questions here along the way.