Check out: http://developer.windowsphone.com/
The package includes (about 350MB):
- Visual Studio 2010 Express for Windows Phone
- Windows Phone Emulator Resources
- Silverlight 4 Tools For Visual Studio
- XNA Game Studio 4.0
- Microsoft Expression Blend for Windows Phone
The website also includes all the details on producing and publishing apps via Windows Mobile Marketplace. Be aware that to publish an app you need to conform to the stated requirements. This involves app testing upon submission.
As far as I have been informed, the marketplace is the only place to distribute apps, you cannot create an app and then just send it to a friend (same as Apple I think).
Enjoy.
Grant
