Make sure to look at the JUCE Demo that comes with JUCE, as well as all of the other example apps. The JUCE Demo has a bunch of goodies in it and can teach you quite a lot about JUCE. The GUI designer in the IntroJucer is pretty great for simple apps, but I quickly found that I can do a lot better when I don't use it. All of the generated code gets kind of messy and you can't change the generated code without it being overwritten. The ProJucer is going to be seriously awesome for designing GUIs and working with graphics, among other things. If you want to change a value in real time (such as the size or position of a Component) while your app is running, check out the JUCE_LIVE_CONSTANT macro. You can see the documentation in the link below.
But yeah, just go through the examples provided and you'll learn a lot about Components and other things in JUCE. That's what I find most useful then I need to figure out how to use something.