So nervous!

I've been playing FFXIV: ARR again lately, and I just hit level 15 with my new Archer on the Brynhildr server, Aria Iris.

And I just got that one quest that makes you join the Duty Finder for Sastasha...

Oh goodness, the anxiety I'm getting right now. I've never once done a dungeon in FFXIV since I've only had one other level 15, and I gave up on her when I had to do this quest.

I've looked up guides for Sastasha, so I know a bit of what to do, I guess.
Kill clams when they're open, and you can't do anything when they're closed, but what do I do with the adds that they spawn? I get that they keep spawning until the clams are dead, so do I just ignore the adds for the tank to deal with until the clams are dead, and then work on DPSing the adds down when the clams are closed and I can't hit them anyway?
And then there's colored coral switches. Do we all have to figure out which one is right and press it, or does only one party member do that?
Same with the things you have to deactivate around the final boss? Is it part of my job to take care of that, or is that more of the tank's thing?
And what are these "treasure coffer" things I see on the map that's listed on the guide I'm reading?
How does loot work in this game? Is a Need/Greed/Pass system like WoW? Is it individual loot? Is there any determining who gets what at all?

And how is this going to change once I unlock Bard? I get that I'll have songs I'll have to use to give buffs and stuff, but does my role change much besides that?

And how do I even get Bard? I keep reading different answers. Is it level 15 Conjurer or Pugilist?

I wish I had kept up with the game since the beginning when we were all newbies. That sounds so much less stressful to me. I'm hopping back into this in the middle of everything, when there's plenty of people who know exactly what they're doing.

And I know I shouldn't be feeling anxious. From what I understand, there's very little difference between these and WoW dungeons, but I'm still anxious! I still get anxious going into WoW dungeons as DPS! Healing? No problem. But DPS always makes me anxious for some reason. I can handle healing. For some reason, I'm confident enough in that that I don't feel like I'm going to mess up and ruin the group. DPS gives me that feeling the whole time.
Oh, and one time, I tried tanking in WoW. Protection Warrior Gnome Tank with Pink Pigtails. Even that was less anxiety-inducing than DPS. Weird stuff.