This is a good discussion, thanks all for participating.
El Phantasmo, STA-21 is an excellent program, and there is nothing wrong with enlisting and then pursuing a commission. But before putting all your eggs in one basket, realize that there other good programs out there that lead to a commission.
The Naval Academy: Competition is very tough and rightly so. You need the grades, extra ciricular activities... the whole package. Basically, you're applying for a full scholarship.
College ROTC/OCS: Again, this means more school.
STA-21: Enlisted Commissioning, as we have discussed.
Don't think that STA-21 is necessarily a "shortcut" to a commission. Some apply for the program three, four and five times.... (that's three four and five years). The best way to a commission is to probably to hold a Bachelors degree. You can get that while in the service with GI Bill or tuition assistance, but unless you devote your off duty time to it, it will take longer this way, and thus longer until you reach your goal of a commission.
Feel free to keep asking questions here, but if you haven't even joined the Navy yet, I would go and speak with an Officer recruiter and ask about different routes to a commission, as anything you hear about here would be a generalization. You could also speak with an enlisted recruiter... but remember thier job is to get you to
enlist.
Good luck!