HTML/CSS is a good place to start as it is super easy and the place where many developers start. Also, it can be used to build web apps & mobile apps. From there, JavaScript will be needed at some point for a web app. Xamarin and C# is a good option for mobile apps as you can develop for both iOS and Android.

I would also recommend avoiding frameworks and libraries to begin with, until you have grasped the language. Start a project and learn while you build that project. My first project was a social network. It taught me pretty much everything I needed to know about frontend and backend development. I think the best place to get started is Udemy as they have some very detailed courses and almost always have discounts, so they do not normally cost too much.

Hope this helps and good luck.