I'm not gonna make an entirely new editing guide, so here are some premade ones:
If you're new to editing, I suggest that you use the visual editor rather than the source editor. This can be accessed by pressing the "Edit" button (not the "Edit source" button). On mobile, it might bring you directly to the source editor; there will be a dropdown menu where you can change to the visual editor.
If you would like to practice editing without changing any actual pages, use the Sandbox.