As a long time healer who has a tank alt in most games, I completely understand the nervousness, I have it too. The tank is the leader of the group, and the one that usually has to deal with most of the mechanics. In this game, I have found that most people dont care up until level 50, thats when some elitism comes into play, since people are looking for smooth runs. Before you get to that point, the best Idea is to tank some lower level dungeons to build confidence. Dungeons dont really get challenging to tank until Brayflox, maybe Halatai too. At that point is where you will really start to build your skills as a tank. The previous dungeons are for getting comfortable with the skills that you have on your class.

So if you really want to train yourself, just climb the latter of dungeons by tanking em. At the lower levels, people are generally forgiving of newness.

Also from someone who gets stressed out by tanking, the best advice is to run a few dungeons with FC members to start.