Which skills and platform do I need to focus as a ASP.NET back end developer? The .NET development environment is so massive, I really don't know from where do I need to start. I do know basic C# syntax and was studying C# 7.0 in a Nutshell and stopped temporarily because it introduced too much concept that I don't need for now. Any advice will be appreciated. Thank you!

Entity Framework,
SQL,
MVC.NET / MVC Core

I can recommend Mosh's online courses, he explains it all really well.

Also ReSharper helps you a ton because not only does it give you a range of super useful features, it also shows you recommendations on how to make your code / formatting better in real time.

Try ReSharper for a month with the trial license, you'll probably be as sold as I am.

@beggarboy thank you so much for your advice! It is helpful for my study plan! :)

@creativeJay Don't mention it. It's super hard at the start just to get a grasp of a language or framework.

Took me long enough myself to get a bigger look around ☺️

For purely back-end, look at EF, WCF, gain some database knowledge e.g mssql, and get familiar with rest api's and how to create them in asp.net,and get really good in C#.

If you also want to do some front end,.dive into mvc as well, experiment with some razor views etc. Asp.net is not only back-end as there is also a clear front end component in it, but you don't have to use that if you don't want to.