Please don't miss out on the fun of tanking because of rude people. Instead of being anxious, realize that you're going to make mistakes and that being yelled at by a rude person on the internet isn't a big deal. They can yell, get mad, and leave, but it doesn't mean anything except that they shouldn't be in pugs.
Forget about them and try again. There are a lot of good people in the community who want to have fun!
Also, if someone is rude, tell them to stop. Even if they don't stop, you stood up for the right thing as part of the community.
Stay positive, work together, be patient, have fun, and thank other players (especially your healer and tank) for playing.

Going into a new dungeon with people who are excited but don't know what to expect is some of the most fun I've had in MMOs. Wiping and then working together to overcome a new challenge is really fun, so don't let anyone take that away from you.