If you are having a hard time designing and making your Windows 8 application look like a true Metro style application, you will find the following resources helpful.
- Metro Design Principles
- User Experience Design Patterns for Metro Style Apps
- User Experience Guidelines for Metro Style Apps
- Design Assets for Metro Style Apps
- Accessing Usability
- Case Studies
Those resources will get you going with your app design, user experience guidelines and base assets.
For development, there are some good documents for general overviews of the architecture and the functionality your apps will work with.
Have fun !