If you are new to GOG then follow these simple instructions to activate your game
Step 2. Login or Signup
Login to your GOG account or alternatively simply create a new account. Enter your desired username, email address and password.
Activate the account via an email if necessary.
Step 3. Confirm Code
Finally, once you have logged in to your account, simply enter/confirm the activation key and the game will be added to your library.
Download the game and play!