There are 3 flavors of Windows Phone in the market today;

1-     Windows Phone SDK 7.1 which is equivalent to consumer facing version Windows Phone 7.5

2-     Windows Phone SDK 8.0 which is same as consumer facing version Windows Phone 8.0

3-     Windows Phone SDK 7.8 was a minor update for devices running Windows Phone 7.5 and adds few features of Windows Phone 8.0 to these legacy devices and can be skipped.

This blog post intends to provide links to required tools to get you started with Windows Phone development given the version your hardware meets or you want to target,

Developing for Windows Phone 7.5 (SDK 7.1)

1-     Windows 7 or Windows 8 as OS (For any issues in installation on Windows 8 please refer this blog post, http://blogs.msdn.com/b/astebner/archive/2012/02/29/10274694.aspx)

2-     Microsoft Visual Studio 2010 Service Pack 1 (http://www.microsoft.com/en-gb/download/details.aspx?id=23691)

3-     Windows Phone SDK 7.1 (http://www.microsoft.com/en-us/download/details.aspx?id=27570 or https://dev.windowsphone.com/en-us/downloadsdk)  

4-     Windows Phone SDK 7.1.1 Update - Not Mandatory but good to have (http://www.microsoft.com/en-gb/download/details.aspx?id=29233 or https://dev.windowsphone.com/en-us/downloadsdk)  

Targeting Windows Phone 8.0 (SDK 8.0)

Note that Windows Phone 8.0 Emulator has specific hardware/system requirements that must be met and can be found here, http://msdn.microsoft.com/en-us/library/windowsphone/develop/ff626524(v=vs.105).aspx

1-      Windows 8 (must be Ready to Manufacturer – RTM) version

2-      Windows Phone 8.0 SDK (https://dev.windowsphone.com/en-us/downloadsdk, Download SDK 8.0 – 1.6 GB)