Stand on your Officer Selection Officer's (OSO) desk until you get accepted into Officer Candidate School. It sucks and is stupid. The instructors are soulless Kool-Aid drinkers who spend all day working out and treating candidates like they are lower than whale excrement. There is an equally stupid process of evaluation called "peer evaluations" which pits candidates against each other. Thus, the politics and groupthink mentality is cultivated early in every Officer's career. Be in exceptionally good physical condition before you go. It is a physical fitness nightmare. Those who espouse a traditional Judeo-Christian ethic will be targeted for elimination, regardless of how good their leadership, physical or mental abilities may be. Officer candidate school is just a job interview. It is followed by 6 months at The Basic School, which is taught by more Kool-Aid drinking instructors who do not work out quite as much or treat the newly commissioned Lieutenants quite as bad; but are still soulless and still full of misinformation. The Basic School will be followed by your occupational specialty school which is progressively better than The Basic School but will still be stupid, full of disinformation, and taught by soulless instructors. Then you will get assigned to your first unit where your first Staff Non-Commissioned Officer will have the dubious task of trying to talk sense to you and undo all the stupidity you have been fed in the year of "training" you received prior to your arrival at the unit. You will have no credibility with your new Commanding Officer because he is well-aware of the "training" you have had. You will be in impeccable physical condition and that is about it. No one will respect you until you pin on the rank of Captain. That rank is instant credibility for Marines in the same way that receiving a college degree is to the academic community. Prepare to be considered an idiot though until you pass that threshold.

Tests, medical, etc I don't remember the entire process so I don't want to do any guesswork. It's long and it takes a few months. You can get more information from a recruiter.