I definitely had the same fear back in TBC. I wasn't just afraid of tanking though, it was heroics in general. I always heard about how hard they were, especially for someone without CC (I was a Boomkin in those days). Of course, WotLK has rolled around, now I have all specs to use. I'm no longer afraid of tanking, extremely comfortable with healing, and meh with DPS. Mainly because I don't find that one as fun. Just don't give up, practice, practice, practice, and you'll do fine. Always be nice, and don't get yourself down if someone is mean.