As Diamndzngunz says read the faq.

To tell you quickly how it works its like this: You have a main char that you play as a usual wow character. All the other chars you play are on /follow which means that you do NOT pass the w, a, s, d keys to them. When your normal char does an attack (sinister strike, wrath, Fire bolt etc.) you place a macro behind the same key (example: key "2") on the alt-chars with something like:

/assist <main-chars-name>
/cast <Alt-class-attack-example-Firebolt>

replace the <> with you specific information. This is the basics for multiboxing, hope you get the principle.

Also define 1 key that means nothing on your mains wow but to all alts is attached to a:
/follow <main-chars-name>
macro.

Good luck