Re power level, you just have to be the right level for certain content or else you don't do damage or die immediately. With changes bungie made though it's rarely a problem as your level is boosted to -5 from the highest level guardian in your fireteam

(Sorry! Yapped there for a bit)

Etiquette-wise, it's really about setting expectations of your experience imo. No one wants to run a know what to do (kwtd) raid and there's that one guardian who has no clue what they're doing. People are willing to teach, but set the expectation:)

If you have a group of people you d2 with, consider going blind into the content and discover mechanics on your own! Otherwise, there are many discord servers that teach. Look for clans that focus on teaching or look at the big beginner focused ones (I think kinderguardians is one)

There are tons of approaches to this! For nightfalls you can match make. Pay attention to the load screen for what champions to expect and bring the right items to stun them. Play into the elemental surges for that damage boost.

With dungeons and raids it depends on how you wana go into them!