I noticed most of the responses here aren't from new users as the OP asked.

So I'll contribute my own 2 cents. Last year I wrote "Methods and Examples for New calibre Users", which you're welcome to borrow ideas from, with the caveat that some of it may be obsolete by now due to changes in calibre and in my workflow. Here's the link:
https://www.mobileread.com/forums/sho...09#post1796609
That shows what I thought was important to new users to know, at that time. In retrospect, it was probably too technical for many new users because I had a technical background, though I was relatively new to eBooks and calibre. During that project I struggled with questions posed in my own mind as well as questions asked by other more experienced folks here:
How new is the new-user audience? Brand-new? Or at my own level of relatively-new-user knowledge and capability with eBooks and calibre? Are their requirements for eBooks and calibre the same as mine? What area is the "new user" new at? There are different kinds of new users, for computers, for mobile devices, for eBooks, for calibre in general. Then there are different kinds of new things in mobile devices and readers, eBooks, and calibre at different levels of expertise. "How new is new", and "what is new to who" are not trivial questions. And the biggest questions in my mind were: Why am I writing this whatever-it-is: list of tips, list of issues and solutions, web help file, etc. Is it for that audience (whoever/whatever) or for me?
Ultimately I realized I was really writing it to help me focus and think about what I was doing with eBooks and calibre, rethink why I was doing things the way I was doing them, and motivate myself to learn enough more that I could change my workflow with eBooks and calibre to better meet my needs. By the time I realized all of that, I'd mostly already accomplished those goals in a sort of ongoing way. So I decided to let that thread die the death of the ignored old thread.
My advice to you is, to avoid your brain exploding into pieces of frustrated and overwhelmed gore, focus on a small subject, a small experience-level of newness, and a small area of using calibre, and focus on audience of users at or below your own level of understanding, on only one issue you thought was important to resolve when you were learning that area of calibre. Narrow the scope as much as possible.
Otherwise the list of issues, tips, solutions will just keep expanding.
Examples:
Scope too broad and probably too technical (IMO): general computer knowledge of how ASCII or Date sorts work. Scope maybe specific enough: how to sort stuff by clicking on column headings, and how to (maybe) set tweaks that affect sorts. Not technically how ASCII sorts work.
Usefully specific scope: How to download news using one of calibre's pre-existing recipes. It's easy, it takes only a few clicks to do, but that alone may require two pages of explanations and instructions that consist mostly of simple explanations: What a news download is. Why it might be convenient to use it. That some calibre users automatically download news overnight, auto-send it to reader, then they read the news on their train or bus commute to work. But ignore the subject of actually writing a new recipe.
Other usefully specific scopes: Any one of the bullets in kiwidude's list or specific areas mentioned by anyone else in this thread. Where they're not too technical. Not all of those issues, scope would be too broad. Just one.