The Air Force's application opens up June 1 (
http://afrotc.com/how-to-join/high-school/deadlines/)
The Army's application ... I believe already opened up earlier this month for high school juniors.
I have not applied Navy, so unfortunate I do not know when the application opens up for you.
Tips and advice:
1.) Get your SATs/ACTs done BEFORE senior year starts -- with all of these Defense cuts & looming sequestration, who knows how much money the Military will allot to ROTC programs. Why am I telling you this? Your BEST chance at a scholarship will be the first board ... the longer you wait, the more scholarships are already given out, which requires you to have a better application for competition. I know, at least for the Army, that the first board is in October. And the first SAT exam is in October - and the test takes a couple weeks to process, so chances are by the time College Board process and sends the scores to Cadet Command, the board will have already convened. See what I mean?
Plus, say if you are not boarded the first time, you can still have more chances at a reboard later on. ... and besides, it's just better to get those pesky standardized tests out of the way junior year. You may think that senior year wont be as bad as it seems, but trust me, you will be BUSY!!! Applying for college, maintaining your grades for college transcripts, sports (presumably varsity), and everything else a teenager does.
2.) Physical Fitness Test - If you have not already started preparing for this test, I would recommend that you start really soon. One - because it's a great set of exercises for your body; Two - because once you start ROTC, you may find that when you tested, the form that you used to do push-ups & situps may very well not be the "proper" way to do so. ... but this isn't as big of a deal as running. If you are not a runner/play a sport that does not require running, I would HIGHLY recommend that you start NOW. I do not know what your physical attributes are, but running can be difficult to fine tune --> it takes time. (well ... for those uber fast runners it isn't, but for some of us it can be). You will find that once you start running, you will not notice changes right away. I tend to find that you only notice improvement 4 weeks down the road. So, don't expect to be running for a week, and then being able to run a sub-5 minute mile.
3.) College visits - When you go on your college visits, always try to set up an appointment to meet the unit (cadre & cadets). This allows you to see if that college's program is a good fit for you. Last thing you want to do is spend 4 years of your life with a program that you don't like. And this also allows the cadre to put a name to a face (and hopefully) get to know you somewhat before the flood of incoming freshmen cadets/midshipmen.
Best of luck!