In Promotion -> Coupon you can create coupon code. In "Coupon Code" text box you can manually create a code of your choice or leave it empty to let system create the code for you. In "Discount" text box enter the amount of the coupon. If you wish to use percentage for the coupon discount, enter percent number (10 for 10%) and check the "Percent" checkbox. You can specify minimum order amount to activate the coupon. If you want to limit the coupon usage, specify the number in "Maximum use" text box.