If you're planning to play Call of Duty: Ghosts then you should plan ahead and decide how you want to construct your build once you're in the multiplayer portion of the game. Much like previous titles in the series since Modern Warfare, Call of Duty: Ghosts sees the player unlocking all of its perks by playing through the game.
Thanks to an early leak of the game's list of perks, we now know when—at what level—each of these perks will be unlocked for you to use in your build.
Interestingly enough, perks can be unlocked at early levels by spending Squad Points, so you won't have to wait until level 50 to get that Stalker perk you have in mind, for example.
As with all early leaks, we caution that the ranks and related perk unlocks may not be final and may be subject to change later on.