I am begging people to learn SQL; it is not a hard language, and it hasn't changed in decades.
Once you have the basics of SQL down (like how to write a query), you want to understand what the database actually does with your queries, so you can predict what the query plan "ought to be"/what you want, and you can read the query plans to try to understand why you did or didn't get the plan you expect.
https://use-the-index-luke.com/
And then once you understand the fundamentals, your database's manual is the place to go. Especially different index types (including their internals!) and what kinds of queries they support. The above link goes into depth about b-trees, but there are other types, and you can use b-trees as a reference point to contrast with others.
I was fortunate to have a Database Basics course in the university that was 80% database schema design and 20% SQL queries and indexes. Reasoning for such a split was that fixing a bad database schema is much harder than fixing bad queries.