This article will cover all elements of running various types of promotions on your website.
Discounting All products
If you are intending on discounting all products (e.g within a certain category, across the site etc), you will need to use a catalog price rule.
Refer to the Magento 2 Community Edition manual here for information on how to do this.
You will need to create Promotional graphics such as banners to inform customers of the discounts and promote the offers.
First create your banner and have it ready to insert into Magento
Then: Create a static block to house the Banner. This section of the Magento 2 manual covers how to create static blocks.
You will then need to create a Widget which controls the logic of where you want the static block to appear. Find out how to create widgets to house your promotional graphics here.
Shopping Cart Price Rules
If you want certain customers to receive a discount on their order then it is best for you to use a Shopping cart price rule. Shopping cart price rules apply discounts to the shopping cart based on a certain set of conditions.
Refer to the Magento 2 Community Edition Manual here for information on how to do this.
You can also see how to create and use Coupon Codes here.
You can choose to promote the coupons around the site using the static block and widget features as mentioned above. Alternatively, you can be selective with the coupon codes and email them to certain customers only (e.g 5% off for repeat customer code.)