Hello.
If you want to provide your customers 10% off promo code on all products, you can create it this way:
1) Go to Promotions -> Shopping Cart Price Rules
2) Apply rule information settings - rule name, active status, customer groups, coupon (or generate it at 5th step) and validity date.
3) In Actions tab select Apply: Percent of product price discount and Discount Amount: 10
4) Save and continue edit
5) Additionally, if you at the first step has not entered the code, you can generate it in 'Manage coupon codes' tab
After that client will be able to apply the code at checkout step -
Here are also examples of other most popular promotions that can be created with the help of price rules -
https://www.mag-manager.com/useful-articles/about-magento-shopping-cart/examples-magento-promotions-...
Hope this helps.