Sounds like you have reached maximum level before you need to prestige. Go into Barracks and scroll down (if you need to) until you see prestige mode. You will lose all perks and lethal/tactical weapons that you have unlocked once you proceed. Attachments and Gun unlocks and levels will remain untouched and will not reset.
There is a permanent unlock token though so use it wisely and unlock a perk/Weapon of your choice that you find most useful to you. You will have to wait until you reach level 4 again to unlock it though. Basically a game or two.
Note too that all your guns will be locked bar the starting ones. Say for example I mainly use the MSMC. When you prestige you will not have access to this gun until you reach I think it is level 18?? Once there you will retain the weapon level and attachments.
So if you prefer the AN-94 for example you will have to work hard to get it again or you have the option to use the permanent unlock token. As P34nut said, use this token wisely.