If you're looking to set up a fully automated online shop - where the money is scooped up for you behind the scenes - I'm guessing you could be feeling dazed and confused. So let's cut the the chase ...
In order to accept credit cards, you need 2 things: an "aquirer" and a "payment gateway". They can be the same company, or not. I suggest a company called Braintree becomes both your acquirer and your payment gateway.
In reality, both the aquirer role and the payment gateway role take a slice of your turnover. It's entirely possible to find yourself shelling out to two different companies for these functions. But Braintree does both for you and that simplifies merchant life.
When you start on a Braintree Standard Merchant's account, your credit card processing costs (except Amex) will be:
$ US Dollar 2.9% + 30 cents
€ Euro 1.9% + 30 cents per transaction
£ Sterling 1.9% + 20p per transaction
For accepting Paypal, you're probably looking at 2.9% + 30 cents per transaction. Dropping to 2.4% + 30 cents per transaction if your monthly Paypal volume is over £6,000 (appox. $8,000).
In addition, to continue being able to accept credit cards, you'll want to maintain PCI Compliance.
Your shopping cart is going to have to integrate with your payment gateway provider's system.
After the pain of integrating: the pleasure of seeing your income in the Braintree dashboard
Integrating with live online payment gateways can get pretty ugly from a programming point of view. With Braintree, I've already tamed the beast in as far as it can be tamed. So wiring you up to Braintree is the most efficent integration I could do for you.