Get to level 65 then your able to Prestige on Xbox Live Multiplayer. The option to prestige will present itself on the left of the menu (where you search for matches etc...)
Level 65- Commander (148680 points needed to reach level.)
To those that don't know your weapons, challenges, level (65 will reset to level 1 and the prestige you are going thru. And you can't go back to a previous level / prestige.
Prestige Up to 9 more times times to unlock a second achievement worth 0 points.
There are several ways depending on how your method of playing or preference is but the fastest way for me to rank up is HQ (Headquarters) You get alot of gun challenges done and kills and with an added bonus of capturing and defending the HQ.
Below are a picture I found on the net about the levels of prestige.
<-- just click on the link and it will show you the different emblems of prestige.