Firstly, you will require an ecommerce solution. I personally use Zencart, but there are a plethora of others. Zencart is free, all you need is a web host with a MySQL database.
After this, you bolt on a payment processer.
I use Paypal and Nochex. Both of these include merchant services. They are roughly the same cost wise. You do not need to be PCI compliant with these, as all the data is done on their servers, and not yours. They route you to an external gateway and send them back once it has been completed. No card details are stored, or even pass through your site.
You will need to set up a business account with PayPal to take Website payments with them, and you need to be fully verified with them. The basic rate is 20p+3.3% (IIRC)
Linky Though if you have a lot more goign through, apply for their merchant pricing to reduce the rates. You only need a merchant account for the larger companies such as Sagepay etc, as they do not supply merchant services, and pp/nc do.
I am getting rid of NoChex. Not much use, and I like to simplify my accounts.
If you have any questions, I can help, as I do the same thing. Can always trust me if you like!