I know this isn't exactly the right place to ask, but I've had no luck anywhere else...
Looking at the courses available, I would probably start with: https://mva.microsoft.com/en-US/training-courses/getting-sta...
then look into either ASP.NET MVC: https://mva.microsoft.com/en-US/training-courses/introductio...
or ASP.NET Core: https://mva.microsoft.com/en-US/training-courses/intermediat...
This is coming from someone that hasn't actually used these, so take my recommendations with a grain of salt.
Alternatively, you can just get Visual Studio and use their "ASP.NET Web Application" template and poke through what is generated, if you want something a bit smaller to start with.
Links:
Umbraco: https://umbraco.com/
Getting Started Guide: https://our.umbraco.org/documentation/Getting-Started/