Krita does not need a tutorial before using. If you don't know about a specific subject about Krita, e.g. reference image, just watch a random video on YouTube.

Also check out their official documentation. https://docs.krita.org/en/user_manual.html

And this community. https://krita-artists.org/

There are also training materials (art, not software) you can buy while supporting the project. https://krita.org/en/shop/