Here's how pre-ordering works.
You wait until 12AM PDT, or 3AM EDT, and scramble to
http://apple.com/iphone.
Then, you select your color, capacity, and carrier.
After that, you select the option to replace a current iPhone on a current AT&T/Verizon/Sprint account. Enter your iPhone's number, your billing ZIP code, the last 4 of your SSN, and your billing password, if you have one. You can change your plan (if you want). Then, you go to checkout, enter your billing and shipping info, and done!
Then, you have three ways to activate the iPhone when it arrives.
1.) Plug it into iTunes, and activate via iTunes.
2.) Activate the iPhone from iOS itself.
3.) Phone AT&T, and have them activate it over the phone.
Then, you've got a working iPhone!