Basic security on websites

Hi people,

I want to learn about basic security issues relating to small business e-commerce sites. This means sites made by people who know a little HTML / ASP / Perl etc, and have set up an ecommerce site. I want to know what issues are the most common, and what you look for to tell.

I am starting right at the bottom, so only basic stuff for now. Are there any good sites which will take you through the fundamental mistakes made by some sites? Does anyone know any questionable sites that i could look at!?
Thanks - Rob