Windows 10 SDK, tools, and samples were released to the Windows Insiders Program website today!  Be sure and start with Visual Studio 2015 CTP6 and the instructions on the dev center: http://dev.windows.com/en-US/windows-10-developer-preview-tools.  Installation order is important in these pre-release kits.

For additional details on what to expect, check out Cliff Simpkins Blog!

Looking to get started with Windows 10?

1. Sign up for the Windows Insider Program
2. Install, or upgrade to, the latest flight of Windows 10 Technical Preview to best support Windows Universal App Platform development.
3. Run Windows Update to check for the latest updates.
4. If you’re running CTP5 or earlier, uninstall your previous version of Visual Studio 2015.
5. Install the tools– both Visual Studio 2015 CTP6 and the Tools for Windows 10 Technical Preview, which includes the SDK, are required when developing for the Windows Universal App Platform. (To install the .iso file, download the files, right-click the local copy, and then select Mount.)