The first thing that you will want to do, if you haven't previously, is to pre-order the game and apply that code to your NCsoft account. I have heard that there are times of peak activity when the codes take an hour or more to activate, so you may need a spot of patience.
Once you have that code on your account you can head over to the name reservation page. The page is working better than when it was a 404 earlier today, but is still straining under load.
My experience was that it was better to go to the main page, log in and then go to the name page instead of trying to log in from the name page.
Once you visit the page and you are logged in that login button will instead say reserve and look like this:
Once you get the reserve button to accept your click. The first time I had no problems, but this time I clicked and just sat there waiting a long-ass time, and eventually it came up. You'll get a pop up window. You can choose a character name and a guild name.
You can see that I have reserved the character name I want most, and I have 8 minutes left to realize I spelled Jeff wrong. I have not reserved a guild name. If I decide I want to, I can choose to do that any time before May 23rd, which is the cut off. Type in a name that you want and hit save. Initially it may say "Editable for ??" which has been pretty much the standard experience.
If you are worried, which I was, just spend a few minutes a bit later and confirm that your name is still in that box.
I hope that this information is helpful to anyone who hasn't battled the F5 monster already.