1. Go to your Library.
2. Click on the spot where it says "All games" or something like that.
3. There should be a thing for "Tools."
4. Click and download Source SDK.
Hint: Never click SDK 2007 or I think 2006 or 2008 or 2009. I forgot.

I downloaded that, but whenever I try to open Hammer it wants me to open a specific game. I do not have any source games. Do I need one?

I STRONGLY suggest you have a look at the Valve Developer Wiki, before moving on:
https://developer.valvesoftware.com/wiki/SDK_Docs

Because either you just create content (maps) for an existing game/mod, in which case you just need that game/mod and Hammer and start, OR you really want to make a complete mod yourself, but you have to keep in mind, that a newly made mod comes with a "very basic" set of assets and entities . . plus you might have to compile the source code first . . .

引用自 britton.sean.web：

I downloaded that, but whenever I try to open Hammer it wants me to open a specific game. I do not have any source games. Do I need one?

Yes. A good way to get around that is to install Team Fortress 2 and use that. Also purchasing other source games will give you more assets to use.

Gotcha. You first need to click 'create a mod', that list wants to select one of your mods not a specific game. I was getting them confused. I am an old hat with the UDK so compile some source code does not bother me. Thanks for your help.

Why wouldnt you need a source game to use source?
dumbass...

引用自 -=Greenhourglass=- {DJ PON-3}：

Why wouldnt you need a source game to use source?
dumbass...

I see we have children posting here. Go to bed. The source engine is advertised as a creative toolset, just as the UDK is. Now hit the sac, school is back in.