In 2005 Dana Spears and I held a workshop on unit testing in Indianapolis. I published some of the findings of that workshop here. At the end of that article, I recommended a number of resources for getting started. That list included Kent Beck's Test-Driven Development: By Example (Addison-Wesley, 2002, ISBN 0321146530) and TestDriven.com.
Given that you asked specifically about JUnit, I would also recommend the JUnit documentation on SourceForge. A great example which walks you through specific tests (follow along in JUnit as you read it) is JUnit Test Infected: Programmers Love Writing Tests. Once you've read a bit about it, the best way to learn is to start doing it. Just start. The more you do it, the easier it gets.
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