I started as a Gladiator/PLD. This was also my first MMO so I had a lot of learning to do. There is also tanking can teach you that you don't get as a DPS, watching enmity levels in the party list, quickly selecting a specific enemy if you loose hate, watching enemy target lines to know where adds will run, etc. I wouldn't say it's necessary to read up on every dungeons bosses before you do them, just say "hey I'm New, is there anything I should know for this boss? " people are usually pretty nice, but if they aren't don't take offense to it personally. If they don't want to share information that will keep you and the party alive they shouldn't be upset if you wipe. Tanking is fun, you should play what you enjoy. I get nervous tanking be content sometimes too, but with experience we learn to adapt on the fly and develope an intuition for what needs to be done without being familiar with the instance. That's what makes tanking so fun.