Easy ways- paypal or wufoo.
If you're on a mac download MAMP; if you're using Windows download WinAMP. Then start MAMP. It will open a PHP MyAdmin page. Create a database called shop in there, then inside shop create a users table and items table. In users create the following fields: id, username, password, email. In items create these fields: id, product, description, price, quantity. Then change your html page into a PHP page (just save as index.php if original was index.html for example). You'll also want to maybe make a payment table with fields of id, credit_card, card_number, expiration, security_code, first_name, last_name, billing_address, mailing_address, phone_number, user_id, and authorization.
Then you need to write the PHP to connect to the database, start a session, output a form that lists the products from your database, and a submit button that sends the item, quantity, user, price, and all the fields for the payment table using method="POST".
You would also need to ensure data security measures as this is sensitive data.
This is the basics. Writing the code will be the hard part but it's all basic PHP that a google search can help point you to the proper, or similar, code.
If you get stuck on something specific drop a line here.