I suggest writing a game. Even if you're not a gamer, cloning Tetris or Pac Man on modern hardware isn't trivial and can help you learn the ins and outs of multiple different aspects of the system, from graphics to sound to file IO. Also, if your plan is to target phones then I highly recommend the Corona SDK. It's in beta right now, so there are still odd issues with it from time to time, but it's a simple API with enough power for most small projects and you'd be able to target multiple phones in the end. Lua is an interesting language, but protects you from a lot of the more tricky bits of languages like C++. There's also PyGame. Python is (for me, someone in the animation industry), the most important language to learn, and it's powerful enough that I can't remember the last time I had to write C++ code.