We have TFS 2015 Update 3 on Windows Server 2008 R2 Standard SP1 and SQL Server 2012. We like to move to online VSTS/Azure DevOps.
For this, we couldn't find any specific document. What we found is that we will need to move to latest 2 versions of TFS i.e. either TFS 2018 Update 2 or 3 before we can migrate to Azure DevOps.
What is the best process step by step to do this?
First of all, check the Requirements and compatibility for TFS 2018 and prepare the hardware according to the requirements.
Create a backup to the current SQL database.
Upgrade the tfs according this detailed guide.
I recommend read this Microsoft article about TFS upgrades.
In addition, you can find here nice tutorial - "Upgrade to TFS 2018 Has Been Done in Production".