I think the first question to answer is .. What do you want to do?
I know a young man, waiting for orders to go over to Iraq .. that started out wanting to be a sniper and ending up a machine gunner in the Marines.
His motivation for joining in the first place was a sincere desire to do something in/to/for the Middle East because of the homocide attacks on us on September 11, 2001.
So the second question answered is .. Why?
After that ... then check out the different branches.