he wants me to dual box him
You can level up the shammy, but if he plays it, be ready for it to be insta-banned. Every char that you play has to be in your name. Every char that he plays has to be in his name. Every transfer has to be from your name to your name. Sorry. Just trying to warn you now so you don't spend all that time and get it taken away.

90% of boxing is /assist and /follow (addons can do follow for you). Assist is just what you think it is, target a char and virtually press assist key (F). I don't target, I focus my main and then:
/cast [target=focus-target] DPSSpell
/cast [target=focus-target-target] HealSpell
You can get fancy if you want, but those should get you started. Really, almost every macro is a variation of those. Start small and work you way up. You will play and find out that you want to do an ability. You spend a little time working out the new macro and go on. Just start small and work your way up.
Hope that helps. Good luck.