C++ is the de facto working language pretty much across anything. at the end of the day though, it's just a programming language--it's not how much you know about C++ that determines whether you're a good programmer, it's how well you can implement your ideas in C++.
and that requires a good knowledge of algorithms. since you're in middle school, you should probably build up your knowledge of algorithms first. everything else is, for the most part, easy and it's something you'll pick it up in college or when doing real programming projects.
so my advice is: definitely try out the USACO (USA computing olympiad) problems.
http://train.usaco.org/usacogate
the above website has a collection of exercises and they bring you along slowly, starting from the most basic. do not be frustrated if you can't solve the problems immediately--just take your time and research on the internet. quite a few of the people submitting solutions to the website are in fact 7th or 8th graders--people like you.
don't be frustrated if you find the problems difficult. by the time you finish the first couple of chapters (out of 5 or something), i think you are already better at programming than most 2nd year computer science majors--in college!
indeed--most of the interview questions companies ask are no more difficult than USACO problems. (regarding algorithms)
the Wii is an epidemic.
