When dealing more then 1 target, open the fight with activating your shield block [right before you charge, I combine it in a macro before my charge ability] and charging, TC and demo shout, then shockwave [can be shockwave 1st, but then you lose the damage shield bonus that does a very nice threat on the pull combined with shield block]. After that just Tab-devestate/revange/ss [when the rule basicly says: SS 1st priority to use, revange 2nd, devestate when both are on CD] things untill TC/SW are refreshed and use those the second you got them off CD. If you're above 50 rage [which could be tricky at times on that level] you can throw in a cleave aswell, to boost your threat even higher.
I strongly suggest you combine Omen or other threat metter into that tab-dance to see which of the mobs is getting attacked by the more threatening members of the group and needs a few more single target hits before you move to your next one.
Good luck with tanking. The best way to work things out is I think Agg's guide. Though it reads for a lvl80 it would have lots of the questions you need answered there, don't miss reading through the discussion as it gives a good sense of different opinions and reasons. I find most questions that have come up for me have been answered already here so I get familiar with the search mechanism, so much so this is I think my first post here despite lurking for months since I made my tank my main again back in January or so