1. Go to
https://secure.square-enix.com/account/ and login.
2. Select 'Select Service' under 'Select Options' and choose FFXIV. Click 'Add Service'.
3. If you haven't before you confirm personal details then enter your FFXIV registration code off the back of the game manual.
4. A number of new options appear along the main menu down the left hand side of the screen, under the heading, 'Payment and Service Fees'. Select 'Register / Update Card information'. It takes you through a procedure that sets up payment authorisation for transactions made on your account.
5. You are taken back to the main menu. For a second time, select 'Select Service' under 'Select Options' and choose FFXIV.
6. Under Active Service Accounts, Final Fantasy XIV is listed. Choose the option next to it - 'Options List'. Next to Character, Choose Add New Options. You then activate character slots. Note you don't pay anything for the first month, so you can choose 8 character slots and not pay anything. Cancel some of them before the end of the first month or it gets very expensive, though.
7. Any bonus item codes are entered in the same way and place as you entered your FFXIV game activation code in section (2) above.
Once this is done you are all set and ready to play when the servers go live.
I'd suggest doing as much of the above as possible as soon as possible (now). It's quite convoluted and the system will get hammered tomorrow.