https://visualstudio.microsoft.com/xamarin/
That will give you a development environment within which to work.
Other than that, my best recommendation is to come up with an idea of a somewhat simple app that you would like to build, of which you can identify the required components/steps. For me, it was a clientserver app to allow me to control the Windows Media Player on my home PC from my phone. So naturally, I needed to figure out how to connect to my pc, launch WMP, get a list of artists, respond to an artist selection with a list of albums, and so on. I used simple text transfer for the client/server interface, so from an android perspective, it was mainly about learning what kinds of objects to invoke to store and display what I wanted, and send and receive commands and data.
I find that it's much easier to learn how to do a specific thing that I know I want to do and why, than it is for me to learn about something in the abstract, without any clear understanding of its use. But I've always had success in learning how to accomplish whatever task I've identified, and I learn about the programming elements involved as I go, and encounter the need to learn about them. And thankfully, that approach seems to work well for me regardless of the programming language.