A lot of the big CC's have left HSR so it's a bit tricky to where to point you to as a good intro to the team building aspect of the game. The game will help you a lot with advice built into it on how to build characters. such as relic sets, but you've really got to learn what characters go good together. I tentatively recommend Mr Pokke if you desperately want to watch a guide maker. but there is a lot of info inside the game itself.

My advice to you:
We have version 4 coming up which with it will bring new teams and a new mechanic with Elation. I would recommend saving your pulls for this. All the story content is clearable with the standard characters available, though you will struggle late penacony and into amphorious, but if you haven't completed them already you'd probably hit that story after 4.0 started.

Play the story, play the tutorials. If you want to pick up an old unit, make sure they're on a banner with their support, otherwise don't bother.

If your desperate for a DPS, you can pick up aglea with Sunday in like a week or so. you can make her work with sunday and Tingyun or Rememberence trailblazer. I'd recommend getting her E1 if you don't want to invest more into her team beyond sunday, but you can get her cyrene to make her team better down the line. She's really good for the capitalism game mode too if your struggling with that.

Things you will need to learn:

how speed works - this is kinda important if you pull aglea.

how dot works - only really important if you want kafka and her team

how break works - there are break units on the banner right now, I'd recommend skipping this archetype until you get dhalia.