Source SDK is a free piece of software available for download by anyone who owns a valve game on Steam, it can be downloaded via the usual steam download/installation wizard.
Source sdk can be found in the tools tab/menu in steam shown in the image below (yours may vary depending on what games you have)
Once you have downloaded and installed source sdk, hammer should be verry easy to locate as once Source SDK is loaded there is a button wich displays as “Hammer Editor” as shown below.
Now you have to double click “Hammer Editor” this will laod up hammer. (First time users may take a while to load while hammer files are configured.) Once hammer has loaded you are free to do what you want and create what you want.
For those of you who would conformation that you have laoded hammer, here is a picture of what it should resemble.
What to do if you don't see this? You have to Setup Hammer by hand.