Yeah, agreeing with everybody posting above ...

You don't have to memorize it all, and it's so easy now. There is so much publicly available documentation. I remember writing code for Macs in the 80's and early 90's before there was a huge public internet. The equivalent of the Win32 API was the Macintosh Toolbox and the equivalent of the GDI was QuickDraw. In order to do anything you had to get these 1000 page books of documentation and by "get" I mean buy, and they were really expensive. Things are a lot simpler now.

But, generally, just write some applications. Know how to register a window class, set up a WNDPROC, and how to CreateWindow an instance of it. It's nice to know how to do basic things with HDC's, to blit from an offscreen bitmap and so forth. Beyond that just look up whatever you need.

Please do not cross-post links to other threads.