In this blog post they point out some resources to get started with Windows 10 development.
Their first recommendation is to watch the 14 part Channel 9 video series A Developer's Guide to Windows 10 Preview.
Join experts Jerry Nixon and Andy Wigley as they introduce the Windows 10 developer platform, give guidance on developing Windows universal and web apps, and take a look at some of the interesting new features for developers in Windows 10.
Once you have done this they recommend going back to the Developer's Guide to Windows 10 Preview and following along with the instructors to learn about Windows 10 and development on it.
These are terrific starting steps but the learning can continue thanks to other Microsoft tools about Windows 10 development:
Microsoft BUILD 2015 is in just a couple of weeks where it is expected even more about the development path for Windows 10 will be revealed and discussed.