There's no native Azure planning tool.
"IT Depends" a lot on what you want to do - but there are resources to assist:
Azure Architecture Center (a lot of the designs, you can download Visio templates for, then adjust to what you want) - https://learn.microsoft.com/en-us/azure/architecture/browse/?WT.mc_id=AZ-MVP-5004796
Microsoft Assessments - https://learn.microsoft.com/assessments/?mode=home&WT.mc_id=AZ-MVP-5004796
Microsoft Azure Well-Architected Framework - https://learn.microsoft.com/en-us/azure/architecture/framework/?WT.mc_id=AZ-MVP-5004796
Azure Pricing Calculator - https://azure.microsoft.com/pricing/calculator/?WT.mc_id=AZ-MVP-5004796
The Azure Rapid Assessment Estimator - could be useful as well: https://usdco.azurewebsites.net/Resources.aspx
You can check out links to various other resources here: https://aka.ms/AwesomeAzureArchitecture (some of the Landing Zone accelerators may be of help)