| Depends what you want from it. Easiest way is probably something like an e-bay shop, but they don't give a very professional image (in my opinion). If you look at Woz's site (kitfondle.co.uk) that is a nice looking site but all the back-end stuff is done through PayPal. I'm sure PayPal will give detailed instructions on what you need to do to link your website to their payment service. I assume you know how to build basic websites already? In which case its likely to be simply linking to the right pages that paypal tell you to link to, but i'm not certain about that.
If you want a bigger "proper" online shop then you can get software off-the-shelf that will do that kind of thing, setup fairly easily and use templates and things, but obiously there is the cost to buy and you will also need to be able to process card payments.
For a small-medium sized shop i'd probably go the PayPal way - look on their website and it'll no-doubt tell you all you need to know.
David |