I would like to join the military but?

I would like to join the US Marine Corps, as a commissioned officer, but I wish not to leave the US. I know it all movie stuff, but I'd like to do something like Tom Cruise in a few good men. Have a job at home but still in the military. How can you do this? And what exactly is the military police under the Marine Corp? What are their jobs, and where do they reside, and do they do any fighting?