So, by now you should have Windows 8.1 up and running.
The next thing is to install the Windows Phone SDK.
A few days ago I tried the Windows Phone 8.0 SDK (it comes with Visual Studio Express 2012) and it works OK.
But VS 2013 Update 2 RC (announced 2 days ago) already comes with the Windows Phone SDK 8.1 (which also allows you to develop for WP 8). This is good, because you don’t need to have two versions of VS installed in your machine in order to develop for Windows and Windows Phone 8.
And yes, in VS 2012 you needed to install two versions of VS: one to develop for Windows and another to develop for Windows Phone. It is like you needed to install two versions of Xcode: One to develop for iOS and another to develop for Mac. Mmmmm….