For all the popular and well-known libraries there tends to be freely available documentation available online or otherwise. There are sometimes books which provide a sort of walkthrough introduction to the topic. For example all of the Windows API is documented at MSDN but that's not an easy way to step into it. A good introductory book (although it might be slightly old now, it's good for basics) for the Windows API is Petzold's Programming Windows.
For this particular application unless you really want to use a GUI you don't really need to know THAT much about the Windows API. Just the registry access functions are required (
Registry Functions (Windows)) and they are fairly simple so long as you have a decent grasp of C.