Looking for help with a site, which plans to sell registration

Hi,

First off, I am not a web developer. I do some design here, and there, and I recently got asked to redesign a website for a foundation that ran some kid’s water polo teams. I can design, but I can’t develop, and the site needs a registration area, where parents can sign their kids up, and enter specific information (like the childs age on a certain date, and their national water polo registration number), when they pay for the registration. The foundation had already set up an account with authorize.net, and they had sent me a guide to do this (Advanced Integration Method Guide) which I do not understand at all. I would like to learn this stuff, but basically the foundation contacted me and said they needed this registration page to be completed by SUNDAY!

So basically, I am looking for help on how to do this.

Thanks,

Harris