When you can create classes and do the simple stuff (GUI, reading text files, etc...), where do I go from here? I've started reading Code Complete 2nd Edition which is great but is more of a general programming book. What topics should I learn next?
I'd argue that, at some point, it's no longer about topics. It's about doing stuff. You gotta write code an you've gotta write a lot of it.
People will often tell you to find some niche problem to try and solve when learning a new language and that's perfectly fine, but if there's an application or feature of an application that you really enjoy using or really admire, attempt to build it (or parts of it) yourself. Even more so, if you find your self wondering Hey, how does [this application] do that?! Try to build it.
Many of your projects may never see the light of day beyond your desktop, but the learning, experience, and tools you'll have under your belt will be something that you can carry over to each subsequent project.
You never know, though, one of those little hobby projects may end up solving a problem for someone.