You say you've done some "basic" testing, but one of the biggest challenges for me upon entering a "real" software development role was adapting to test-driven development. Even if you don't take this all the way and write up all of your unit tests before you start coding, you do have to completely re-think how you structure your software so that it can be testable at all. This means breaking up your functions not just into parts that make it more readable, but into parts that can be independently tested. Usually this means breaking out your dependencies into interfaces, so that they can be mocked out (oh, and learning how to mock things), avoiding side effects, that sort of thing.
Also focus on writing readable code. I usually make one or two passes after I think I've gotten everything written and refactor with an eye toward making everything readable and understandable.