Can you help me with this Java issue?

I’m using OS Tiger for the Mac and I keep getting “Execption in thread “main” java.lang.NoClassDefFoundError”. I Googled it and it said to change the class path parameter, but I have no idea what that means. Also when I Googled it, I found that this is a problem lots of beginners have but it mentioned no way to fix it.

How can I find the Java development kit on my Mac? (10.4.x) If it isn’t on there already, where can I find one on the internet?

Thanks!