To get started with B2B integration, you will need an Integration Account—this is a special container for integration artifacts you will work with. In general, it allows you to store a variety of different items (certificates, schemas, and agreements) in one place, so you can work with them in Azure Logic Apps.
To create such an account, click on the + Create a resource and search for Integration Account:
As you can see it offers a simple form where, in fact, the only thing you need to do is select the proper Pricing Tier.