I don't think anyone should have to read a guide or watch a video before stepping into a dungeon, but it is common courtesy to say it's your first time even if no one asks you (and if someone does ask, say yes!). It takes less time to explain the fight than it does to wipe repeatedly until you can figure out what the problem is. If you think people will get upset with you because you say it's your first time, imagine how much more pissed they'll be if they wipe a bunch of times because you decided to keep quiet.