First you go to server browser.
Then you pick a server.
Then you pick a gun.
Then you shoot your gun at other people.
You get points for hitting the other people with the stuff you shoot out of your gun.
Points+More points = Rank up.
Do that over and over and you will be prestige 15 before long.
But if you are talking about a hack to get you there, try reading the other 700 closed threads about this.