Lately, I've been going through the process of learning about Valve's Source.
Initially, you should own a valid copy of any Valve games (I actually own HL2, so I am ready to work).
First step in this process is to get the Source SDK from the Valve website. It's really easy, as you only have to open the STEAM Control Panel and, under the Tools tab, you have to double click on Source SDK Base and Source SDK packages, and Steam will install and download them for you. As easy as it can be.
The follow link will guide you in detail:
Please follow the instructions ... not as I did ... It's mandatory that you run the game before running the SDK, otherwise SDK won't run!
When you are ready, you can launch source from Steam Control Panel. Your entry point then is to visit the SDK Reference Docs located in the Documentation Area.
What's first? Creating a Map? Creating a Mod? Is there any place to start? I am lost!!! Here is where I complain about the documentation from Valve. Well, According to the Oxford English Dictionary (1997), complain: say one is dissatisfied. So, instead of complaining, I will try to give some more details on the tutorials where you can get clueless as there are not details, screenshots, etc.
So, let's start. The following link will take you to the Entry point for basic documentation:
Next tutorial will guide you through the creation of a Mod.