Before you join the military, ask yourself if you are willing to give your life for your Soldiers (or Marines) and your country. If you can say yes to that question, then you are ready to join. Many do not consider that question, and then are surprised at the risks they are required to take in time of war. Although, the conflicts in Afghanistan and Iraq will change and eventually end, enemies of individual freedom continue to exist all over the world. These fanatics will continue to attack the United States and her democratic allies, until we change the dynamic of world-wide poverty and provide opportunity for all.
Now, if you decide you still want to join the military, and you want to be an officer, I recommend you join a reserve officer training program. You can join anytime during your college education. During one of your summers, you will be required to take a college version of basic training for either Army or Navy ROTC. This will give you the initial training and understanding of the responsibilities of wearing the uniform, as well as some of its rewards.
I am not a Marine officer, so I cannot tell you what it is like from a personal perspective. However, my friends that are Marine officers are very proud of their service, and expect high standards of behavior from themselves and their Marines.
If you choose to be a Marine, you will need to take Navy ROTC, and apply for entry into the Marine Corps. After you graduate and are commissioned as a Marine second lieutenant, you will take Marine basic infantry training. All Marines must take basic infantry training before they are qualified to take any other Marine specialty training. You will not be given your desired specialty until you prove yourself in your initial training.
You can also join the Marines out of college, but the path will be more difficult. You will have to go through enlisted basic infantry training. If you prove yourself worthy there you can earn the chance to go to Marine Officer Candidate School. Here you earn the right to become a Marine second lieutenant. You would already be at this point when you graduate, if you complete a 2 or more year ROTC program.
Unfortunately, you cannot not join the military for a few months. If you are commissioned, your commitment is usually at least 3 years duty. If you accept a scholarship, or a monetary entry bonus of some type, your commitment is usually 8 years, mixed between active duty time and reserve time. The nice thing about trying ROTC is that if you really dislike it, you can turn in your uniforms and stop attending. You would have no binding commitment, unless you accept a scholarship of some kind.
Please talk to the ROTC program at your college or at the university closest to you to get more questions answered. There should be an OTC program close to you, even if your college does not have one. You should be able to join as a member for a satellite campus.