How do I enable line numbers (on the left) every time I open Emacs?
Drop (global-linum-mode t)
into your .emacs
if you want it for every buffer. Otherwise, you can add a function to the appropriate hook of modes you're interested in to enable it for just that mode.
You should really read though the manual like I suggested in the last question of yours that I answered. ;)
Since Emacs 26, the new way is to use (global-) display-line-numbers-mode:
(global-display-line-numbers-mode 1)