When you edit an item in the Regular interface, you will notice a lot of new fields. Under the Price field you will see another field labelled Sale Price. Try entering a number there and clicking on Update. You should now see a reference to the regular price and the sale price.

The sale price is the price that is used by the ordering system if someone orders the item.

You can change the text that appears before the regular and sale prices. It is controlled by the variables Regular-price-text and Sale-price-text, on the variables page.