I touched on this on your original post in the correct section for account issues.
If you're activating YOUR account from the buddy key, you just need to pay for the game and start your subscription. Unless you bought an actual game card, you'll need to choose one of the monthly payment types.
Choose 1 month plan to start, enter your credit/debit/whatever payment method. It will then set up billing and you can play for 30 days before it will bill said payment method.