For several years, you would access Hammer by going on the tools section in your steam library, then running the Source SDK. However, shortly after converting HL2:DM to SteamPipe, Valve changed this.

Hammer is now run via a batch script inside your HL2:DM folder, the exact path to this folder is:
steamapps\common\Half-Life 2 Deathmatch\bin

Inside this folder is a file called "hammer.bat". When you run this script you should be presented with a small graphical menu, one of the options will be "Half-Life 2: Deathmatch".

EDIT: It is possible that you do not have a hammer.bat, and only have a hammer.exe. If this is the case, you should just be able to run the hammer.exe, and also receive the same graphical menu.

Unfortunately, mapping is beyond the scope of this tutorial.

In short, do NOT try to create maps by starting hammer through the tools menu in Steam anymore, as this method is no longer supported.