Before i explain, i want to say, that this may sound ridiculous, but its what im going for, and no matter how much you flame, it wont mean anything...
a) You need to learn to differentiate a flame from honest advice. Most kids come in here wanting, but not listening, thinking anything that goes against their desires is a "flame". It isn't.
Nobody told you to never build an MMO. Some of us believe that an MMO is a very, very large undertaking, esp. if you are serious about a career and not a dilettante. I am one of those. Start small, make you inevitable design mistakes on simpler things. Do you learn to swim by jumping in the deep end first? Do you learn to compete in tennis by playing against Agassi first? There is such a thing as biting off more than you can chew...
i am hoping for a professional career as a Game Designer, but i dont know what i need to learn, and because of my problems at school, i want to start learning early, and that way i also spend my time at the PC, for something usefull.
It's great to be 14 and know what you want to do! Most kids don't.
However, there's no simple guidebook to becoming a game designer. The short and sweet really is to become a game designer, you must design games... of all sorts. You must understand how rules can clash, what makes a game good or bad, etc. You must focus on math (statistics and game theory primarily) and some social sciences (political science and psychology, primarily), but breadth of experience is very important. Travel, read different styles of books, etc.
BTW, the best book that would give you a global perspective on what it is to be a game designer is Jesse Shell's book. Also, Fullerton's is a good read too.
Will Python, C++ and any other coding language, be usefull in the future?
Will Python itself be useful in, say, ten years when you are fully into the job market? Unknown. Will learning Python be useful? Of course. It's been said a million times on this site: a good developer likely has been exposed to multiple languages; it's knowing the algorithms, and when to use each one, that matters most.