How to activate an iPhone ordered online?

I currently have an iPhone 4S with ATT and would like to upgrade to an iPhone 5 while keeping me same number. How difficult is that to do yourself if I order an iPhone 5 from Apple.com rather than going to an ATT store? I NEED to keep my current number though.

Just call or take the phone into the AT&T store and they'll port it from one phone to the other in about 2 minutes.

Just bring it to AT&T and they'll connect it in a few minutes for you.