You'll want to start off with navigating to your tools library on steam, and downloading Source SDK (NOT Source SDK Base).
If you want to map for TF2, You'll also need TF2 installed.
From there, there are a lot of different tutorials around the web to get you started (
a lot of them, are here, on this website.).
The first place I suggest you check out, is this article on the Valve Developer Community wiki. The VDC is like your all-needed reference manual for the technical stuff of Source.
***I'll get you that article soon, the VDC is surprisingly down...