For example, we can open a new buffer with C-x b <new-buffer-name>, and by default, this new buffer opens in fundamental-mode, which is not very useful to me. I would like to change this to open all new buffers in org-mode. How can I do this?
You have to customize the major-mode variable (see also the emacs manual). C-h v:
Symbol for current buffer's major mode.
The default value (normally `fundamental-mode') affects new buffers. A value of nil means to use the current buffer's major mode, provided it is not marked as "special".
You can do this with M-x customize-variable major-mode or with something like (setq-default major-mode 'org-mode) somewhere in your init file.
See also initial-major-mode for the *scratch* buffer.
