Starting out: There are some pre-made behaviors I still use, but otherwise I agree with SadiQ's advice on them.
I don't like the advice that's sometimes given to avoid other people's behaviors, extensions, and other code. For one thing, it's contrary to modern software development principles. You don't start from scratch. You use libraries and engines. You're using Stencyl, aren't you? That's someone else's code

But I understand why that advice is often given. A lack of understanding of programming will be a significant, if not the most significant, limiter of how far you'll get developing games on your own. Working in a team? Then the limit is how much the best programmer on the team knows their programming and their ability to communicate with everyone else on the team.
You can do a lot with pre-made behaviors, even if they're not always enough to make a complete game. But have a good attitude about them. At the start, keep the scope of your projects small and make learning a priority.
Making the transition from beginner to intermediate: Learn to write re-usable code. Don't cram all your code into the actor's events; write behaviors that can be used across multiple actors and scenes. Better yet, write behaviors that you can also import into your other games.
Learn how the kit behaviors work--even the complicated ones. Unless you plan to start your game from scratch (as in using no engine at all), you need to learn to dissect other people's code. Everyone has their own approach to things. When you're able to interpret other people's code, the kit and pre-made behaviors become a lot more useful--because you'll be able to modify them when needed to suit your specific needs.
Extensions: Learn how to use the extensions that are available and how to write your own.
Math: Algebra and trigonometry are very useful across multiple types of games. Most people learn these in high school, so if you've graduated make sure you're on top of them. If you are still in high school then take these subjects seriously.
Making the transition from intermediate to advanced: You need to learn how to analyze algorithms. How do you measure timing and storage requirements? Learn what 'Big O' notation is. Learn how to write efficient code. This will save you unnecessary worry about whether your code will lag, and help you write complex code that still runs at 60FPS.
A large, complex program can be thought of several smaller programs working together. Even as a beginner working on a first, small project this is true: you're letting the Stencyl engine do almost everything and only a small bit of the code is yours to worry about. Working on a larger-scale project puts more code as your responsibility and less as Stencyl's. It only takes one poorly designed procedure to bog down the entire system, so knowing how to analyze code for efficiency is a must.
Learn to use a debugger and analysis tool (like hxScout).
Learn how the Stencyl engine works and how to modify it.
Math: Vector algebra and linear algebra are a must if you want to do advanced work in graphics or physics. Discrete math is a must if you want to build complex AI for your game. Calculus is a must if you want to write a physics engine that models motion and gravity like Box2D does (or understand how Box2D works and how it is limited).