The only key commands an Emacs user must memorize is:

:wq or ZZ

Probably preceded by pressing ESCAPE, to get out of vi's input mode.

Once in (X)Emacs, you're inside a "self-documenting editor", so you don't need to know any of the commands. If you find yourself in the editor for the first time ever, just read the splash screen; it will tell you all you need to know. C-h t is very handy, if you're too lazy to read even that.