Which payment gateway/processor should I use?
Choosing a payment gateway can be a bit of a daunting prospect as there are a lot of options out there.
If you haven't had an ecommerce website before and are choosing a gateway for the first time an obvious option is PayPal. PayPal is very easy to setup (you can have a working account within just a couple of minutes) and many people like to pay through PayPal as they believe it offers them some protections that they don't get with other payment methods. Your customers will very likely recognise and be familiar with paying through PayPal.
Alternatively if you are looking for a more substantial gateway you could try Barclaycard. Barclaycard is probably the most suitable payment gateway for most online businesses, they have some great rates available and are simple and straight forward to setup.
However, you may also like to look into alternatives like SagePay, WorldPay or contact your bank and see what gateway they offer.