Although I never served, I have nothing but respect for those who have and do serve. I have several friends who are former Marines (there's no such thing as an ex-Marine!), and love the fact that they chose the Corps. Others, to be sure, wish they'd chosen differently. I assume that you have done your research, but if not, know that each branch has their own 'personality'. The Marines probably have the highest sense of esprit de corps; they do more with less, and are eager to tackle the toughest jobs. As their officer to enlisted ratio is so low, you will tend to be given more responsibility earlier than with some other branches. A number of the military folks I've talked to from other branches have commented on the competence and professionalism of the Marines they've worked with. The Air Force, on the other hand, has a very high officer to enlisted ratio, but spends the most money and effort to ensure quality of life for their members. There's a reason that the other branches refer to the AF as 'country club'. That may or may not be a bad thing from your point of view. In short, I've known people from each of the five branches, and most all have been proud to have served where they did.
I'd recommend that you take the ASVAB, talk to recruiters (early in the month - don't catch them when they're scrambling to make quota!) about what jobs are available and you're qualified for (and remember that they'll train you - just because you don't know it now doesn't mean that you can't learn it!), check out some military blogs - there are some very good ones from most branches with active and helpful communities. If you need me to point you to a few, just PM me. It's a big decision, make sure you approach it carefully and responsibly, but you have my respect for being willing to serve!