How to find more help:

More help than you will ever read is started by pressing Meta-H or function key F1 for help. Press F1 twice (or M-h twice) to get help using help.

M-h m Describe Mode lists all the key commands for the current mode. For example, if you are looking at your files list in dired-mode, M-h m will tell you to use D to mark a file for delete, E to edit that file, and many other file commands.
M-h a This runs "Apropos" and asks for a word. It gives you a list of all commands which mention that word. M-h a save will list all commands which include the word "save" such as save-all-buffers.
M-h k Describe Key is more useful than you think. It asks you to press a key and then describes what that key would do if you pressed it. M-h k C-x C-x says "exchange point and mark", whatever that means.