That's what the wiki is hopefully going to become.
In the mean time, there's a "help" file (limited) that comes with later versions of the program. And the "readme" for the latest versions usually documents new sysvars as they are added.
Other than that, the sysvars2.21.text file that comes with later versions contains all the sysvars the program has to use. Just scan it at some point and post questions about ones you see that make no sense to you.
There are less than 200 sysvars I believe, so it's daunting but not undoable to sort through them all.