If you service select for Marine ground in fall of 1/C year, you will put in for your TBS class dates in the winter, same time as classmates going Navy are getting their assignments to ships, homeports and aviation/submarine school pipeline dates.
You turn into a Marine on Commissioning Day, take some leave and head off to TBS at Quantico for your designated class date to receive the training all Marine officers go through. During that time, there will be plenty of briefs on various professional options to pursue as a Marine ground officer. That's your MOS. There will also have been some briefs during your spring semester as a 1/C, once you know you are headed to the Corps.
Like everything else, the "needs of the Marine Corps" will rule the day, in terms of how many slots open up for each MOS during various TBS classes moving through the pipeline.
Once you get your MOS assigned, you may then be assigned to additional schools after TBS specific to that MOS.
During your time at the Academy, you will have the opportunity to watch and learn as upperclass go through the process. You will talk to Marines assigned to the Yard as company officers, senior enlisted leaders and academic/athletic/professional development faculty. You might join Semper Fi as an extracurricular activity. If you are dead set on Marines, you will definitely want the Leatherneck summer training block during 1/C summer. The Marines on the Yard do a superb job of modeling their service and helping midshipmen understand the mission, tasks, functions and culture of the Corps.
I've found this website to be helpful over the years to me as a naval officer, though it's not an official usmc.mil site: http://usmilitary.about.com/lr/marine_corps_officer_job_descriptions/327476/1/
Lots of interesting fields to pursue...