oh you're going to have hours of fun...
it sounds like you want to use php programming code that works with an sql database.
i started with "the idiot's guide to php and sql" which is quite basic but does the job. Then now I've got hold of the basics I use "PHP and MySQL Web Development" as a reference book. Although it's often faster to just do a google search for what I'm looking for.
credit card processors are set up to deal with php - you send variables to the payment page, and then the credit card processor sends back the variables to your order processing page with notification of whether the payment was successful or not. The security is taken care of by using md5 encryption (it's easy and the credit card processor will give you instructions) and a 'shared password' that will stop other people getting access to your order processing page/script.
cheapest way to get setup is paypal but if you're doing any kind of volume then worldpay is the way to go. you also get a slight uplift in sales because people trust the worldpay logo (i've heard 10-20%). it'll be expensive in the first year but you can negotiate with them the next year.
so what's the big idea???
Good luck,
MrS