You can software box, with several toons on one computer (or even across a few computers), or you can hardware box with one warcraft per computer. Both options are good, depending on what you want.

Most of us are software boxing.
There is a sticky in the software forum, which talks a bit about the various options.
I'll recommend IS Boxer, as easy to set up, powerful and flexible once you want to adapt your system to be better able to beat an encounter or whatever.
But that's hardly the only option.

How many toons are you interested in?
Many go with five toons, for a full party.
But two toons is the minimum to be dual-boxing.
And some go with ten, for 10-man raids.
A couple (Sam & Prepared) have 30 odd accounts.

Are you interested in PvE heroics or PvP melting faces?
Or both aspects of the game?