Process for buying online?

[qwertyasdfasdf]

Could anyone tell me how buying a phone online works? I'm on a family plan (an Optimus S), and one of our lines (not mine) is available for an upgrade (which I'm looking to use to upgrade my phone! My father doesn't want a new phone anyways, haha). So... if I order the E4GT online while renewing that line's contract, what happens? Do we have to activate the new phone before using it (and specifically, will the current phone on that line still be usable until the E4GT is activated?) Is the new phone tied to that line automatically, or could I activate it directly to my number?

Edit: Maybe this might fit better in the Sprint section? But it's an E4GT I want, and I figured a good number of people had probably bought their phones online, so...