warriors will have to aggro dance until they get shockwave.
thunderclap works and theres a glyph somewhere that will rend multiple targets. forget the name. these can be used. i dont know too much about warriors, just a warning.

anyway, you forgot to mention levels, as some folks may have level 60 warriors ready to go via RAF, but others may mean lets start a warrior at level 1. i'd like to know what level you're starting on so we can better give you advice. whats your team composition like? just three shammies and a healer?

just like a druid tank, always give the tank some time to build aggro. they have rage to use in order to do their abilities, and rage starts with a charge, then some autoattacks/getting hit does the trick to add more rage. tab through a few targets. what i find useful is to put a HoT (heal over time) spell on the tank before a pull. this ensures that the warrior's healing draws aggro to himself and not the priest/druid that casted it. as your tank begins to build aggro with the rest of the mobs, you can toss another HoT when the other expire. always keep HoTs up and throw in large heals when necessary. by the time you toss the first big heal, you should be in no danger of losing aggro to your healer. on bosses, begin the same way buy you dont have to worry about multiple targets losing aggro. i would begin my dps/healing after two sunders. this is what i do for my druid, to begin dps after two lacerates for solo bosses and after the second swipe and first mangle for multiple mobs. with pallys or DKs, you simple dps right after they drop a consecrate or DnD. rage-dependent classes dont have that luxury.