This seems to be one of main purposes of this subforum. Hello!
Let me tell you some cool things.
If you like a post, want to thank it etc, there are rating button in bottom-right corner of each post, so you don't make useless posts about how you agree.
Be sure to check out content packs at bottom of the main page. My favorite is the Construction one, and the new war themed - Frontline! - is awesome too.
There are lots of tutorials on lots of stuff. Video and not.
Other sites you can find useful articles at are
Nodraw (this one's cool, not that many articles but all are good),
Facepunch (mostly around GMod mapping, but technically there is much useful). TopHATTwaffle has YouTube tutorials.
His website.
@Crash has cool series of literal map making tuts, but there are many other things you learn, like publishing a download, updating it, getting it tested.
Well k most of this was obvious ofc but eh