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To start, I'm going to point you to the tools and resources you'll find at Windows Dev Center. After that, I recommend the videos from Microsoft's last BUILD conference. Beyond the basics, I can also give you some advice that might not be immediately obvious to you from reading the docs. That's what the rest of this article is about.
Know Your Options
Architecture of the App
Picking a technology is only one piece of the technical puzzle, however. You also need to be careful about how you structure your app. As an industry, we've moved from client-only to client-server to n-tier to our current favorite: hub-and-spoke. The hub-and-spoke architecture provides the basic structure for most modern mobile apps: At the hub, you've got your server, which provides data and logic synchronized between multiple client spokes, such as Windows Phone, Win8, Web, iOS, Android, etc.
Each spoke is designed and optimized for the specific host OS and the hub is the authoritative holder of the data shared between all clients. This kind of architecture calls for several considerations, including communication channel decisions (pull over HTTP, push over SMS, etc.), offline caching support, per-client data projection and filtering, and so on. As each spoke is added, you'll find that the hub often needs to change to support the particulars, so you'll need to think about the user experience you want and make those hub design decisions accordingly.
Specifically, when it comes to Windows 8, you'll want to tailor a hub-and-spoke client for things you've undoubtedly already heard about: a touch-centric UX, Win8 UI style, fast and fluid animations, and so forth. What you might not have heard about is the three primary rules for a high-quality Win8 app: integrate, integrate, integrate! In prior versions of Windows, we had the Clipboard, which allowed each app to share whatever data it wanted in whatever formats it used. In Windows 8, there is still a Clipboard, of course, but it is only one of many "contracts" for sharing data between apps.
For example, the SkyDrive app not only allows you to browse the files in your SkyDrive account, but it's also a File and Folder "provider," so when you want to open a file from a Win8 app, the file and folder dialogs know to let you choose something in SkyDrive as well as on your local computer. The app that's requesting the file doesn't need to know or care that the file is coming from SkyDrive the operating system takes care of that, in the same way it maintains the Clipboard. Other contracts include Search, Settings, Share, and Contact Picker, just to name a few.
In addition to the contracts, which allow you to integrate your app with other installed apps, the Win8 Start Screen also enables a great deal of integration with Live Tiles, Badges, Notifications, and the Lock Screen. Live Tiles are especially important, as they represent a key differentiator of the "Metro style" UI shared between Windows Phone, Windows 8, and the XBOX.
And speaking of differentiators, another key thing you should do in your app is just that: differentiate. When you read the guidelines that describe Win8 UI style, you'll hear a great deal about the Windows 8 way of arranging data, using the contracts, providing the right animations, consuming the right gestures, using the right fonts, and so on. However, once you've gotten your head around the Win8 UI style, you'll need to bend or even break some of the rules so that your app stands out from the crowd. Your app must have personality as well as functionality, if it's going to be featured as a top app in the Store and get you the reputation and revenue it deserves.
With Windows 8 and the Windows Store, we're at the beginning of a new era of Windows, which brings devices, touch, and UX design to the foregroud while keeping desktop apps running well, too. The future is going to bring great changes as we learn how to design apps for this new world. Stick around. We're just getting started!