Activation code

If the game is for the PC, there is nothing you can do. Serial keys are 1 time use only and the person you bought it from will have known this.

If the game is for a console, you will have to purchase a new online pass for the consoles market place. It says this on the box of the game.

OP if you purchased the game used from Game Stop look at your receipt, the online pass code should be on the receipt because Game Stop now gives out free online pass codes for used games that require them after they lost a recent court case.  If you purchased the game new the online pass code is on the back of the instruction booklet or on a separate sleeve of paper inside of the game case.  It also may be worth trying to type in the online pass code from the game case even if it's used because not everyone uses their online passes and you may get lucky.

If you purchased the game used from somewhere other than Game Stop then you will have to purchase a new online pass for $9.99 USD on the PSN store\Xbox Live Marketplace\Origin Store depending on your platform.