To make emacs automatically use auctex when a .tex file is loaded add
this to your .emacs

  ;; Load Auctex when a .tex file is loaded.
  (autoload 'tex-mode "tex-site.el" "auctex" t)
  (add-to-list 'auto-mode-alist '("\\.tex" . tex-mode))

Here is the manual BTW:

  http://www.gnu.org/software/auctex/manual/auctex.html