You have three options for setting discounts. It can be based on subtotal, customer groups or both.
Before adding discounts to Customer Groups, you will need to create them (if you don’t have any- you can do this by updating the Customers page).
For discounts based on subtotal, once you have added the discount it will automatically be calculated at checkout.
For Discount Coupons, go to the Discount Coupons section under Marketing category and select Add New Coupon. A discount coupon is different form a discount because it requires the customer to physically type in a discount code to receive the promotion.
After adding the new coupon and communicating it to you customers, they can then add it by selecting Redeem your coupon.