This is just going from memory of the dozens of similar threads to this I've read, but let's see if I can help some people out...

Firstly, to the OP, I apologise for the general community on these forums, as 90% of them have absolutely no reading comprehension.

If you're from NA: Go to www.findmyorder.com enter your purchase order number that was sent in your confirmation e-mail and your square enix password to login, from there, there should be a way to get your 18 digit code which will give you your 20 digit code. Something like that, you can work the rest out.

If you're from EU: You're SOL. Ordering from the official EU SE store seems to be totally pot luck, some people have received their codes instantly, others haven't gotten a goddamn thing, and no amount of support tickets will help.
Some people have reported cancelling their SE order and ordering from Amazon instead and getting their code, but, there's no way to instantly cancel your SE order from the EU store, you have to lodge yet another service ticket and ask to cancel your order, so we'll see how that goes. On the plus side though, once you get your code and use it, feel free to cancel your amazon order too.

If you're from Australia: You're even more SOL, as Amazon UK doesn't deliver to Aus and there's no digital download option.

To find where to enter the code, simply try and log in with your launcher, it'll tell you that you have no service account and give you the link to the mog station. Mog station isn't up until the 27th, so all you'll see is a message telling you that, and a link to the website to enter your code.

And if you have a code and are wondering "WEER DE EFF R MI IN GAEM ITEMS!?!!?!!!11". They will be delivered to you via the moogle mail after OFFICIAL launch on the 27th, now please stop entering these threads and asking stupid questions, because the people that can't play the game at all want to rip your stupid head off and shove it up your arse.

Lastly, if none of this helps you at all, you're probably either A: SOL, so go cancel any orders you made solely for early access, try to calm down and just wait for official launch, or B: stupid.