Create a Web App within an App Service plan from portal
You can create an empty App Service plan from the App Service plan browse experience or as part of app creation.
In the Azure portal, click New > Web + mobile, and then select Web App or other App Service app kind.
You can then select or create the App Service plan for the new app.
To create an App Service plan, click [+] Create New, type the App Service plan name, and then select an appropriate Location. Click Pricing tier, and then select an appropriate pricing tier for the service. Select View all to view more pricing options, such as Free and Shared. After you have selected the pricing tier, click the Select button.
Create a Web App within an App Service plan from powershell
New-AzureRmWebApp -Name $webappname -Location $location `
-AppServicePlan $webappname -ResourceGroupName myResourceGroup
Create a Web App within an App Service plan from powershell (RM Based)
New-AzureRmAppServicePlan -Name ContosoAppServicePlan -Location “South Central US” -ResourceGroupName ContosoAzureResourceGroup -Tier Premium
Create a Web App within an App Service plan from Azure CLI
az webapp create –name $webappname –resource-group myResourceGroup –plan $webappname