We explicitly label easy first issues in our issue tracker: https://github.com/bevyengine/bevy/issues?q=is%3Aissue+is%3A.... These should get you up to speed with the contribution process and, at the very minimum, get your feet wet with hacking against the engine, if not dive deeper into various specifics.

I strongly suggest any of the docs issues, as it doesn't require you to write any actual functionality, but forces you to get to know the architecture, the language, and the core parts of the engine itself.