How to add a custom processing code in wordpress?

My question can look trivial, but I'm new to such t开发者_如何学Chings in Wordpress.

I've an already created cformsII, I need to process it manually to send a mail, pretty simple thing, then I need to use some of the data to redirect to a payment gateway.

How could I add such processing page into wordpress?

Does it has something to do with "module" or "extension"?

By the way, is there any best practices to follow / know when doing such things within Wordpress?

Thank you

If you are going to redirect to payment gateway, i suggest use http://www.s2member.com