Microsoft's powerful open source development platform .net core is the new trending technology in the tech market, so if you have visual studio 2015 you can do the development in easy updates.
So I will be explaining the steps required to configure the .net core in visual studio 2015.

1. download the visual studio update for 2015: download
    If you don't have the visual studio 2015: download
2. Download the .net core tool for the visual studio: download

I hope you have downloaded and installed all the above items, now we are good to go to the next step
So let us create a hello world console application using .net core
Select the new project option from vs you can see the .net core option under C#
select the console application and create one project with that

now you can just write the code in the same way you used to write with C#

The DLL generated in windows machine for this application you can run in Mac/Linux it will give you the same result.

you can check more details here :details

As discussed in my previous post(Read), you can create mobile applications for all major platforms using c# and .Net
There are 2 IDE's available for Xamarin development, you can use Visul Studio or Xamarin Studio
Xamarin studio come along with Xamarin installer if you are using mac this your IDE, for windows users you can download the Xamarin for visual studio from below link.

Download Xamarin
Xamarin developer version is now free along with visual studio 2015.
Download :Visual Studio 2015

If you want you can use the different emulators,  by default along with Xamrin you will get android emulator, but its little bit slow.
Here is one third party tool you can use for android emulators, It will support from windows 7 onwards.
Genymotion Emulator : Downlaod

Another good tool is Hyper V Emulator : Download but there are some compatabilty problems with windows 7 so those are having windows 8 can go with this emulator.