I wanted to start Cross Mobile App Development for that I had heard Xamarin is a good framework if you are good with C# , so , I had downloaded the latest Visual Studio 2017
First off welcome to the Xamarin world.
Secondly your templates that you have are correct. They simply changed up a bit in visual studio 2017. For instance, if you want to create a PCL ( Portable Class Library ) project, you select cross-platform app and click ok. Than you get following screen:
Here can select your coding strategy ( I chose PCL in screenshot ).
If more questions don't hesistate to ask and goodluck ;)