PHP as CGI on hosted shared server

What do I need to do to run my PHP script in CGI mode? It is a portable script (in this case it simply takes form variables and uses the mail function to send the info to the client’s email address, which is also passed as a form variable). I want to use the same script for various clients whose sites may be hosted on a variety of servers.

Any help would be most appreciated as I am new to the deployment side of development and the issues therein.