ARM templates are JSON format documents that declaratively describe the structure of a resource group and provide an infrastructure-as-code solution to your Azure environment. With ARM templates, you can deploy a full environment into your Azure account in a repeatable way that is source-control friendly. 

You have a few options for creating an ARM template:

1. Write it from scratch. The syntax and schema definition for ARM templates can be found at https://docs.microsoft.com/en-us/azure/azure-resource-manager/resource-group-authoring-templates.
2. Create it with Visual Studio by using the Azure Resource Group project:

3. Add resources to the ARM template using the JSON Outline tool:

4. Export a resource group from the Azure portal:

5. Use a pre-made template and modify it to your needs. Microsoft offers a catalog with many templates that you can use: https://azure.microsoft.com/en-us/resources/templates/.