I never thought I'd be asking about this, but what are some good ways to get started with the e-commerce/shopping cart/credit card validation crap?

In lieu of getting a real job, my girlfriend and I have begun designing web pages. I'm the IS graduate, and she's finishing up her Graphic Design degree. Anyway, one of our new "clients" has asked about starting a web shop after we design their initial marketing site. I've mostly programmed in PHP, dabbled in perl and ASP, and would like to learn ColdFusion, but I have no idea where to start with e-commerce balogna. I haven't promised anything to them yet, because I want to make sure I can do it (get something up and running) before I take on that kind of responsibility.

I'm looking for some direction as to what are good ways to validate credit cards (PayPal?), what are the primary security concerns with doing something of this nature, and is one scripting language better suited than any of the others? Basically, if anyone else has any experience, what would you recommend?