My team was in the same situation as you about 2 years ago. We started writing tests for bugs that were reported by customers, as those were the "hot-spots" for

My team was in the same situation as you about 2 years ago. We started writing tests for bugs that were reported by customers, as those were the "hot-spots" for regression. As we became more confident and adept at using JUnit, we started writing tests for new features too. Only after 6 months did we start looking at places in the code that were not covered by unit tests and started filling them in to avoid regression. It was a lengthy process to get our coverage to a decent level, but it was definitely worth the struggle.