Regarding the windows 8 designing, Microsoft is providing a portal with a lot of amazing resources around the windows 8 modern UI guidelines and how to develop your apps. They are also providing a lot of case studies and use cases of how can you reimagine your existing website as a windows 8 app or an iPad application as a windows 8 application, further more you can find samples of how to design a sports app or a news app etc. you can download a PSD to start designing your app from here. You can also find a lot of modern UI icons at the following websites: thenounproject.com & thexamlproject.com, but I would prefer the xaml project as you can download your icons either as PNGs or as XAML which is better because it will give you the ability to resize them from the code behind without getting pixelated but on the other hand the noun project is gone provide you with SVG icons which can be resized with your image editor without getting pixelated. Another resource for modern UI icons is adamwhitcroft.com/climacons but it only contains weather icons.
If you know any other resources I’ll be glad if you shared them with me in the comments.
For other resources to get you started you can visit this post.