Right, you are in for a world of joy then. This little guide is if you dont want to build everything yourself.
a) You'll need a shopping cart (one that integrates with the following services) (interspire, x-cart, cubecart, zencart)
b) You'll need a payment gateway that takes money from Mr and Mrs X and puts it in the bank. (PayPal, sagepay, Barclays ePDQ)
c) You'll need a bank with merchant account facilities to accept incoming monies. (Barclays, HSBC, PayPal, WorldPay)
b and c cross over a bit. Services like World Pay and Paypal can provide merchant services and the payment gateway.
If I were you, I would look at Paypals business services, website payments and payflow. Key differences arise whether you want to keep people on-site for the whole transaction or send them to PayPal hosted pages for the payment and then back to the store.
I would advise using their hosted pages as this avoids complications with PCI DSS (new legislation for online shops about the security of customer details etc).
If you get more specific questions just fire them back here and I should pick them up.
