Well, unless you want to get involved in the murky world of maintaining a
SSL (a secure link) to take payment (with the security issues & considerations
of storing credit card details on your machine) I'd suggest getting a company
to do it for you. Try:
www.worldpay.com

In it's simplest & cheapest form, you make the shopping cart (which you said
you've done) and they look after the credit card side of the transaction
and put the money in your bank (eventually i.e. a month). Does save a lot
of hassle though....

Hope it helps - Scott.D