Shopping Cart Assistance Set Up Needed

In selecting an item to add to the cart, the system reports that we have successfully added the item. However, when reviewing the cart there is a $0 balance. It is shown in one spot that the cart is filled, but in another it shows that it is empty. When I go to the selection that reflects the filled cart nothing happens. What would be a basic plug in/set up needed in order to have a shopping cart system that works and that allows orders to be placed? We would greatly appreciate guidance that would allow us to solve this situation that is preventing customers to place orders.