wow, I admired for having such an ambition in a young age.. coz teenagers nowadays doesn't really think about their future.. all the think about is enjoy, party and stuffs.. it's like they never know when they gonna die, may be tomorrow so this is the time for them to get high and stuff so they will not regret it in the future.. haha/
my advice is to try your best to hold on with your ambition.. your chances of becoming a mangaka in japan is quite harsh at the moment, but who knows.. one day maybe you make a new style of manga that will make you popular.. japanese people will cherish you like your the next hayao miyazaki etc..
good luck mate..