The best place to start for all things related to WP7 development is the App Hub (http://create.msdn.com/en-US/). From here, you can download the WP7 SDK via http://create.msdn.com/en-us/home/getting_started. The SDK will install the necessary tools as well as the emulator to test your WP7 applications on your PC.

At the time of writing, the newest version of the SDK is 7.1. This version allows us to build applications for Windows Phone 7.5 (also known as Windows 7 Phone Mango) and 7.0. This chapter contains recipes that are specific for WP7.5. Where this is the case, this will be clearly indicated. The 7.1 SDK release is fully compatible with the 7.0 version. You should target the 7.1 SDK for every new project.