This is where you can pick your Military Ocupation Services.

Aviator
Personnel & Administration
Intelligence
Infantry
Logistics
Artillery
Training
Tank and Assault Amphibious Vehicle
Ground Ordnance Maintenance
Ammunition and Explosive Ordnance Disposal
Signals Intelligence/Ground Electronic Warfare
Linguist
Data/Communications Maintenance
Supply Administration and Operations
Traffic Food ServiceManagement
Financial Management
Motor Transport
Morale Welfare and Recreation
Public Affairs
Legal Services
Combat Camera
Recruiting and Retention Specialist
Music
Chemical, Biological, Radiological, and Nuclear (CBRN) Defense
Military Police and Corrections
Electronics Maintenance
Aircraft Maintenance
Avionics
Aviation Ordnance
Aviation Logistics
Meteorological and Oceanographic
Airfield Services
NavigaPilots/Naval Flight Officerstion Officer/Enlisted Flight Crews
Pilots/Naval Flight Officers

To Narrow down your MOS put in this link

http://en.wikipedia.org/wiki/List_of_United_States_Marine_Corps_MOS