APIs

I want to get into software development. I know the various languages most companies use, and new languages are fairly easy to pick up. The problem I have is working out what APIs and interfaces I should be learning. I've spent time poking around MSDN, but it's a huge place and I've no idea where to start. What are the key things I should learn about .NET, WPF, ASP and the like, and what are good tutorials/ guides/ books you would recommend to learn them?

Start with driver interfaces, protocols and file formats. Everything else consists mostly of bloated frameworks and junk, with an occasional specialized utility library.

Easy: libCurl.
Normal: POSIX.
Hard: Win32, Direct3D, OpenGL.
Lunatic: GTK+.

Extra: X11.
Phantasm: ALSA.