All the pre-order items are novelty. They are not going to include an item in the game that gives players a superior advantage over other players based on where they purchased the game.
Same with those who bought lifetime subscriptions and other such things, there are slight bonuses, but nothing that will unbalance the game.
To get your pre-order item(s) you will first need to enter your retail key when you receive it.
Then you log into the game and access the C-Store. From there there should be a list of qualifying items for you to claim. It'll be one to a character.