C-x h will select the entire buffer.
You can search for help within Emacs using the built-in help system.
C-h f will look for help for specific functions. In this case, you could have searched for whole-buffer to find mark-whole-buffer.
Other help commands of interest would be:
- C-h m to show available commands for the current modes
- C-h v for help related to variables
- C-h k to find which functions keys are bound to
- C-h w to see which key bindings are defined for a given function
- C-h ? to see which other options are available.
Note that C-x h will only highlight all the text. To actually copy the selected text, you must use C-w for cut (kill-region) or M-w for copy (kill-ring-save).