MOS selection and availability?

Hello Marines.

I'm curious about the selection process of an MOS. I read in another thread I found (could not post there) that an enlisted cannot select an MOS, but that reservists can. From the way the conversation went I could not get a clear understanding. Is this true? How does the actual process work? How much of a say does a DEP'er have in what he or she gets? I am also curious about the availabilities of MOS'es. I know that times change and the Marines have needs in certain areas and are over manned in others. Does anyone have any idea of which MOS positions are more difficult to obtain? Maybe someone can point me in the rigt direction to find that information. What I am all about is the intelligence field. That is really where I want to be in my life.

Thank you for your help.