make a macro and keybind it out of the way:

Name it SetFocus
/clearfocus
/focus MainToonName

Then for your spell's on your ALT:
/cast [target=focustarget] spellname

That is the most common way of targeting and assisting.
You will want to add TAB to your Do-Not-Pass list as well as your movement keys if you havn't done so already.

If your looking for a more basic way of casting and don't want to mess with focus's yet try this:

/assist MainToonName
/cast spellname

That is probably the most basic spell assist macro, but does have some flaws.