Perhaps the single most important thing I've learned in my development career is the concept of separation of concerns. You'll soon realize though that once you begin to draw distinct boundaries between areas of responsibility in your code it quickly becomes much more modular.

At this point I am 8 or so years in to my development career. There's a vast amount of practices and methodologies I know about development now that I obviously did not then. Here's a list of things I wish I had known sooner than later.