UM1718 is the 451 page user manual, and is pretty good.
Like many GUI tools, videos are often the best way to learn how to use them. It is awkward to say "click this button", but many videos move too fast for you to see what buttons they are clicking.
This "getting started" video from ST is pretty bad. And it is 7 years old. The sound level is too low and the guy moves too fast. He seems in a hurry to "get done" rather than to explain things. The appearance of the IDE has changed a lot, but if you want a whirlwind tour and are willing to dig for details later, it is only 10 minutes to suffer through.
Like any IDE based tool learning is a mix of exploration with the tool itself and reading various tutorials and resources and thus accumulating knowledge.
Tom's Computer Info / tom@mmto.org