To cause Emacs to automatically go into Smalltalk mode when you edit a Smalltalk file (one with the extension .st), you need to add the following lines to your .emacs file:
(setq auto-mode-alist
(append '(("\\.st\\'" . smalltalk-mode))
auto-mode-alist))
(autoload 'smalltalk-mode "path/smalltalk-mode.elc" "" t)
where path is the path to where the two Emacs Lisp files included with gnu Smalltalk are installed (by default, something like /usr/local/share/emacs/site-lisp).