Yay. I thought I was the noob here.
You'll need a toolkit called "Source SDK", which you get when you buy Source-engine games (CounterStrike: Source, Half-Life 2, Half-Life 2: Deathmatch...). Then, simply open up your Steam and choose Source SDK from the Tools list. You'll now see a list of tools, all of them are somewhat useful, but you'll want the Hammer Editor. Double-click on it and start building your Source maps.