Printable cheat sheet for popular modes
Emacs ships with cheat sheets of common and useful key bindings. The pdf files are located in the refcards/ subdirectory of.
Editing key bindings
You can add, remove and change key bindings for any command, either globally or locally to a mode, interactively (and only for the current session) via M-x global-set-key and M-x local-set-key, or persistently using simple Emacs Lisp code in your Emacs configuration file which is loaded when Emacs first starts up.