Convert Website to Ecommerce

My interest are in converting my current website to an E commerce website where retail purchases can be made securely. The site is complete. I need to have a simple method for a user to purchase 1 of 4 product options in any quantity. Then I would like the user to purchase the product through the site with a credit card.? ## Deliverables I would like to explain again, this is only the conversion of an existing website to an e commerce website.