Start by selecting a cart ( free or paying ):
http://www.pdghosting.com/shopping_cart/carts.htm
THen, you need to see if you can interdace your payment processor with it.
The rest is just to skin the cart: either a designer made to measure one, or buy a prefabricated one...
I have a few for clothing, but I do not have the time now to search for them.
If still interested, I could show you tomorrow.