The answer depends a bit on where and how you pre-ordered the game. If you pre-ordered it from the Playstation Store or Xbox Marketplace digitally, then the pre-order bonus items should be digitally delivered to you automatically, and you don't need to do anything at all. Just check your inventory once you have gotten into the game.
If you bough the game physically, you will need to redeem the bonus items manually. The exact method depends on your console, so skip to the one you have below.