Learn how to debug your code. This is not just about the tools (research and get familiar with the appropriate tools for your language and platform) but also about writing code that is easy to debug and your mental model for how you approach debugging.
Think about how you can test the code that you’ve written — both at the micro-level and for the entire application. Get into the habit of writing your own test cases, or rather, generating your test cases. Can you be comprehensive? (why not?) What are the corner cases? Each time you make a change / add a feature / fix a bug, ensure that your code passes all the tests you have defined.
Learn how to talk about your implementation: the choices you made for the data structures and algorithms and why you chose this instead of that. And remember:
Debugging is twice as hard as writing the code in the first place. Therefore, if you write the code as cleverly as possible, you are, by definition, not smart enough to debug it.
Brian Kernighan