You can create promo codes in your store under Customers > Groups. If you go there and look under the Help tab, you'll see a pdf Promo Manual that you can download to help you get started.
Once you've set up a Customer Group and established how members are added, you can create Rules that will apply to anyone who is a member of that group. Rules have both Triggers and Results. Entering a promo code is considered a rule trigger, while receiving 10% off the order would be an example of a rule result.
Watch a video on how to set up Customer Groups and Rules.