After you received your key (get one here: https://www.gamesrig.com/buy-final-fantasy-xiv-online-starter-edition-na-cd-key), you can start registering and download the game by following these steps below:
1. Download Final Fantasy XIV Online installer here https://gdl.square-enix.com/ffxiv/inst/ffxivsetup.exe
2. Run the installer and select region as North America
3. If you don’t have a Square Enix account, choose “I do not possess a Square Enix account.” to start creating an account. Otherwise, select “I currently possess a Square Enix account.” to login.
4. Choose your Country as United States. Accept the User Agreement then click Next.
5. Fill in your personal information then click Next
6. You’ll receive an email confirmation code. Fill the form with the code then click Next.
7. Your account is now created and ready to use.
8. Fill the form with your code purchased. Accept the User Agreement then click Next.
9. The screen will confirm the activation of your code. Click Next.
10. The launcher will start downloading the game. Once it’s finished you can start playing right away. Have fun!