You may need to add a coupon code to your website to provide a discount to your customers:
Login to the shopping cart administration
Click Tools & Settings > Coupon Manager
Enter the details of the coupon including the code to use for your coupon, the type of discount whether percentage or fixed, the total discount, and dates.
That's it! Now when customers add an item to their shopping cart they will have the ability to enter a coupon code as well.