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)