To activate a Blizzard game key, you will need a Battle.net account (you can create one here). Follow these simple steps:

1. Go to Blizzard and sign in to your account.
2. Click on your email address located at the upper right corner of the page, and then select “Account Settings.”
   
3. On the Account Settings page, select “Account Overview” from the left-hand menu.
   
4. Paste your game key into the designated area and click on the “Redeem Code” button to complete the activation process.

If you have any further questions, you can search the Battle.net FAQ for more information.