shopping cart blues

[Mberrynk]

I am designing an html site for a small business who recently told me they want to add a shopping cart to it. To this point I've only designed static html sites but have wanted to get the ecommerce experience under my belt. She's asking me what 'default gateway' we are going to use (is this simply paypal, etc?). Can anyone tell me how to go about setting this up(the cart, certificates, scripts)? I'll need the basics. I know this is a lot of info but hopefully someone is up to wearing some letters off their keyboard to help out an up and comer.

RIML