You can develop Windows 8 Metro Apps using the Windows 8 Developer Tools/SDK .
This includes the necessary tools and SDK for developing the Windows 8 Metro Apps like Visual Studio Express 2012 for Windows 8 , Microsoft Expression Blend for Visual Studio 2012 for Windows 8 .
At the time of writing this article , the Release Candidate of Microsoft Visual Studio 2012 was available which can be downloaded from Visual Studio Product website
You can download Windows 8 Developer Tools from Windows Dev Center