Juan Céspedes 

If you want to make GNU Emacs fullscreen, there are three things you should do:

• Disable tool bar
  This can be accomplished executing (inside Emacs) (tool-bar-mode -1)
• Disable menu bar
  This can be done executing (menu-bar-mode -1)
• Go to full screen mode
  You have to execute (set-frame-parameter nil 'fullscreen 'fullboth)

If you want to disable always tool bar and menu bar, like me, and you want to be able to go to full screen with a keystroke (F11, for example), add this to your .emacs:

;; F11 = Full Screen
(defun toggle-fullscreen (&optional f)
  (interactive)
  (let ((current-value (frame-parameter nil 'fullscreen)))
    (set-frame-parameter nil 'fullscreen
      (if (equal 'fullboth current-value)
        (if (boundp 'old-fullscreen) old-fullscreen nil)
        (progn (setq old-fullscreen current-value)
          'fullboth)))))
(global-set-key [f11] 'toggle-fullscreen)

;; Disable tool-bar
(tool-bar-mode -1)

;; Disable Menu Bar
(menu-bar-mode -1)