You should try picking up Actionscript if you want some game design experience. C/C++/C# is good for a final product that you wanna get out (downloadable titles). But... like I said, AS is light and really portable, so if you can host stuff online, have more people playtest (since Flash has such a large installbase).
Prototyping is an important game design skill, where you basically create a little throwaway project that highlights the main gameplay/feel you want - if the prototype is fun then you can try doing a full out project. Since Flash is so light (and there are little libraries and things you can pick up for free online, like Flixel), it makes for a great prototyping tool.
anyway, I'm still learning (flash/vgdesign concepts/gameplay issues) too, so iono. I'm kinda interested in what you end up doing/making, so get back in contact when you start making stuff x]