First thing you will need a shopping cart program. There are some good cgi and php ones on the net. Some are free, while others can set you back some cash. It all depends on what you are looking for.
It would be best if you had a secure server (SSL) to do business on. You can spend $100 (a year) for a secure certificate. You can also rent space on a 3rd party secure server.
If you do a lot of transactions you will want to look into merchant commerce. Basically its an online bank. When the customer 'submits' their order, the merchant commerce will automatically charge their credit card. All you have to worry about is shipping the order. If you don't plan to do a lot of business, you can use PGP key to encrypt the customers orders, and then mail them to you. You have to manually process the credit card and order yourself.
Hope that helps...