Basically the first thing you need to do is organise an Internet Merchant Number with your bank (Make sure they don't give you another Merchant Number as these won't work).
Then you'll need to get a secure payment processor - Worldpay are quite popular, but I use SecPay.com - they're a smaller outfit but very helpful and cometitive rates. Your bank may also offer this service (All the main High Street banks do).
Lastly you'l need to connect your site with the payment system. This can get very technical depending on the level of complexity you require, but most payment processing companies offer help with this.
Hope this gets you started in the right direction.
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