If you plan on tanking, I would REALLY reccomend getting imp TC. I'm levelling doing only instances (not the best way to level, but it's fun and I'm doing it with a RL friend who plays a healer), and even though I'm going prot I'm not 0/0/X I'm 8/0/X to get imp TC.
As far as holding threat...get leader so you can mark targets. Spamming TC and keeping up demo shout SHOULD keep threat off the healer (especially since heal threat is then divided amongst all the non-CC'd mobs), and that leaves you to put the heavy threat (sunder/devastate, shield slam, revenge, heroic strike IF you have a lot of rage built up) on the target that the DPS is going to drop.
A few other tips...if you have enough threat built up on the #1 kill target to keep it on you until it's dead, go ahead and build up threat on the #2 target (may take some time to get used to though).
At the high levels, bring plenty of competent CC. I know the DPS warriors will hate me, but if your group consists of DPS shammies, warriors, priests (unless it's an undead mob), paladins, or druids you will have a harder time tanking due to not having enough CC. On 5-mob pulls if you can have your crowd controllers (priests on undead, rogues to some extent on humanoids, mages on humanoids and beasts, warlocks on humanoids, and hunters on anything but frost elementals and some giants) reduce the pull to 2-mobs you will have easy-mode tanking. In fact, if you bring a hunter who's worth his salt you can drop it to 1-mob (just pick up one of his targets next).