Just ask specific questions in the right forum section 'general or new multibox users' and people will gladly answer your questions.

To answer the two you asked:

Yes you need an account for every character you intend to play and the expansions as you level. You can buy vanilla WOW and multibox up to 60, then buy TBC and goto 70, etc... Or just buy them all at once- regardless you need to buy the game for each character you plan on playing at the same time.

You do not need 2 monitors and you don't need 2 computers either. Programs such as ISboxer create small windows at the bottom of your screen for each WOW open.