Add the following lines to your .emacs file. The last three lines define global keys for some commands — please choose suitable keys yourself.
;; The following lines are always needed. Choose your own keys.
(add-to-list 'auto-mode-alist '("\\.org\\'" . org-mode)) ; not needed since Emacs 22.2
(add-hook 'org-mode-hook 'turn-on-font-lock) ; not needed when global-font-lock-mode is on
(global-set-key "\C-cl" 'org-store-link)
(global-set-key "\C-ca" 'org-agenda)
(global-set-key "\C-cb" 'org-iswitchb)
With this setup, all files with extension ‘.org’ will be put into Org mode.