As I recall, the first time you do something that needs Java the OS will ask if you want to install it, and version 6 is what you get. According to this page, typing java -version in the Terminal is enough to trigger the prompt.
For the JDK (and not the runtime), you’ll need to log into developer.apple.com, search for java, and download the Java for OS X 2012-006 Developer Package. I just did this on my MacBook and now have javac 1.6.0_37.