It’s currently a bit difficult to find good resources for Windows 8 App development.
Do you search for WinRT or Metro? Or even Windows Runtime?
Using XAML leads to WPF results or WP7 stuff.
Anyway here are few good resources:
General Support:
Free consulting services
31 Days of Windows 8
http://31daysofwindows8.com/days
Programming:
Windows.UI.Xaml.Controls classes (WinRT)
http://msdn.microsoft.com/en-us/library/windows/apps/jj203560.aspx
UX:
UX guidelines for Store Apps
http://msdn.microsoft.com/en-us/library/windows/apps/hh465424.aspx
Some good Stackexchange Answers
Design Templates