Understand programmatic approaches to testing, unit testing, test driven development (TDD), behavioral driven development (BDD), and integration tests.

Understanding TDD and practicing it as a new developer forces you to understand the end result wholly. It's one thing to understand how to solve a problem, but understanding how to validate that the problem is solved programmatically, before you have implemented the solution makes you a better developer. It gives you a better view of what you are doing and will change you way of thinking about solving problems.