When i first started tanking back in Wrath i was going through the same thing. I was fortunate enough to be taken under my GM's wing (the other tank). What i did learn from her was that in raids not to worry to much because along as you know you did all you could to hold aggro and survive then you'll be fine, but if you do lose agro and cant get it back just say something to the other tank so hopefully they have something to help you out. But aggro now is way easier to hold than it was back then you really don't have much to worry about
Just remember to keep TC on CD, tab through targets keeping shield slam on cd, and revenge when ever possible, and devastate if nothing else is up. You could also get Omen Threat meter so when you tab you see what mob needs more work on.