In App Service, an app runs in an App Service plan. An App Service plan defines a set of compute resources for a web app to run. These compute resources are analogous to the server farm in conventional web hosting. One or more apps can be configured to run on the same computing resources (or in the same App Service plan).
Select your Subscription and Resource Group using drop-down.
Name: Give a name to your app service plan
Operating System: Windows (For Linux users, select Linux).
Region: Any region
Sku and size: If you are developing just for testing, I recommend you to take Shared infrastructure under Dev/Test. You get free tier with 60 minutes/day compute. For production environment select pricing tier under Production.
Very informative.