UNSC Rankings are based on how to act in a combat situation and the time spend in UNSC service. Note that these are only Marine Corps ranks, there are separated ranks for Army, Naval and Air Force. Note also that you will need to ask me for a permission if you want to be Captain or above.
Private
http://images.wikia.com/halo/images/0/09/PV1_%28USA%29.png
Corporal
http://images2.wikia.nocookie.net/__cb20080815015053/halo/images/c/cb/Cpl.png
Sergeant
http://images.wikia.com/halo/images/4/4d/380px-USMC-E5.svg.png
Lieutenant
http://www.halopedian.com/images/9/97/O-3_insignia.svg
Captain (Ground forces)
http://www.halopedian.com/images/9/97/O-3_insignia.svg
Major
http://www.halopedian.com/images/4/4c/O-4_insignia.png
Lieutenant Colonel
http://www.halopedian.com/images/a/a8/O-5_insignia.svg
Colonel
http://www.halopedian.com/images/c/cf/O-6_insignia.png
General
http://www.halopedian.com/images/7/78/O-10_insignia.png