Under Purchasing → Suppliers you will find an overview of all suppliers.
To edit an existing supplier, click on the respective supplier number or name. To create a new supplier, click on Create supplier in the top right-hand corner of the supplier overview and fill out the form. Each supplier is automatically assigned a number. You can adjust the corresponding number range under Settings → Shop → Number ranges → Suppliers.
This article explains how to assign products to your suppliers and how to store further supplier-specific data, such as purchase quantity and price.