To give a percentage or dollar amount off of an order's total, you can use the Amount Off Order option.  In addition to assigning the Price Difference, this discount can also be assigned minimum and/or maximum purchase requirements and can include a valid date range.

If you do not want the discount amount taken off as products are being added to the cart, you may opt to use a Coupon Code that is entered during checkout. A Coupon link is also created and can be given to customers for promotional purposes.