Welcome to Azure! Licensing is a more advanced topic that you can likely skip for now until you are more familiar with setting up VMs. While a Windows 10 VM does require a license (such as a Visual Studio subscription), you do not need a license for a Windows Server VM as it is included in the cost of the VM.
When you are setting up your VM, leave the "Would you like to use an existing Windows Server license?" box unchecked (this would be used if you had a qualifying Azure Hybrid license which is outside of the scope of an Azure 900 course). There is a step by step walkthrough of a basic VM setup in our VM quickstart (pay special attention to step 8):
https://learn.microsoft.com/en-us/azure/virtual-machines/windows/quick-create-portal
Also note that 750 hours of Azure B1S General Purpose Virtual Machines for Microsoft Windows Server is included in the 12 month free Azure account:
https://azure.microsoft.com/en-us/free/free-account-faq/
Additional information on Windows 10 licensing for Azure: