Depending on your college situation, you go to Officer Candidate School for what can be either 2 six week increments or (this one I forget) one 10 week increment I believe. You must be selected by a board that goes through your local Officer Selection Officer.
You put the package together which includes your test scores for the PFT (physical fitness test) and Aviation Aptitude Battery tests if your going into the aviation program. OCS is NOT easy. Unlike enlisted boot camp, they WILL send you home if you don't adjust.
They are evaluating your leadership traits. You will be first and foremost a leader of Marines and they want to be sure your capable of that. You will run, and run, and run, and..... oh yea, run. PT everyday, get verbally abused everyday, and have to show leadership even through all of that.
If you graduate OCS and obtain your commission, then it's 6 months of The Basic School (TBS) when you get your degree, and that is a whole different animal. Hope this helps. And by the way..... Run