A good and free turnkey online store solution is Oscommerce http://www.oscommerce.com/
FYI If you're not familiar with software installation, Godaddy offers this product in their hosting products along with a bunch of other things like Wordpress, Joomla etc. This way you can get one installed easily.
I have been using Oscommerce for a while and this is why I mentioned it. Feel free to look at the others http://hostingconnection.godaddy.com/Home.aspx