I wasn't in the Marines, but I was in the Army Rangers. My advice to you: pick an MOS that you enjoy, doesn't make you too miserable, and hopefully something that applies to the outside world. If you are actually interested in your military career--Pick something like aviation, medical, or computers. You get normal work hours, chances to go to college, and you don't have to sleep out in the fucking rain!!!!!!!!!!!
Me, I joined to bust down doors and jump out of planes. And I did a lot of it. But, when you get out, like most people do these days (cause the Pentagon is soooo concerned about retaining quality NCOs), putting "jumping out of airplanes and killing people" on your resume won't get you a job (normally). That's why I am in grad school now.