Apple has chosen to do it's own implementations of java for the OS X platform. Your system should have java, javac, jar, etc. at your disposal without you doing anything. You shouldn't worry too much about the version if you're just starting out, what you have (Java 5 or Java 6) should be fine for you. I might be wrong about all of the java programs being installed by default, but if you install XCode from your OS X DVD, or from here:
http://developer.apple.com/technology/xcode.html
You'll have to sign up for a free ADC membership for the download.
To check, just fire up terminal and type 'javac' and see what you get.
-Lee