JUnit testing is fairly easy once you understand the in's and out's. In my view it is a more flexible way of using assertions, so if you have never used assertions perhaps familiarize yourself with these.
Programming With Assertions
Next, get familiar with the API. Depending upon which version you are using, knowing a bit about injections might help
Annotations (The Java™ Tutorials > Learning the Java Language > Classes and Objects)
There are tons of tutorials out there, and Eclipse has a built in JUnit framework that makes creating and running test cases much easier