Talk to people you know that were/are in the military. There aren't many movies that can convey to you the way it really is.
Don't trust what the recruiters tell you. That is something you will hear a lot researching your choice. They have a quota to meet, and they will promise you the moon.
If the MOS you want isn't available when you go to sign up, wait. That recruiter will call around and pull all his strings to find an open spot.
Don't go in the Marines just because of what you have seen in movies. It is very infantry based. If you want a technical job, the army may be the place for you. If you want to fly, look into the navy. If you want to sit on your ass, check out the Air Force.