Hi Brian--
If you look in the online help, it has pretty good information on how to use callbacks. Look for the page on the doc_add_callback function (help 148 on the command line), and the general page on callback functions (help 762).
You can put the doc_add_callback() instruction in one of your startup ACL files. See Startup command files in the online help (help 5716).
Basically, to put it in a nutshell, you'll have something like this:
doc_add_callback(0,"create",your_function_name);
in one of your startup script files, either in your custom/init directory, or your doctype directory (if you only want it to apply to certain doctypes--in which case you'll also need to change the first parameter as described in the help). See the "Startup command files" help page cited above for help deciding where you should put this code.
--Clay