Nice :3
The proportions seem to mostly correct to me. As you surely noticed, the hands don't look perfectly natural. (Who doesn't have that problem too? ^^') If you want to improve that, then I suggest you to draw hand poses by looking at photos. Yeah, that seems uncreative and boring but with the time you will get the right fealing for drawing them. Of course it's the same with every other body part.
The poses are quite dynamic but unfortunately also standard poses. But I don't think that this has to be an absolutely bad fact because it's really hard to find poses which look individual and also interesting.
I also think that the picture would look more powerful with colors, especially because it's such a happy motive.
But my opinion is that the chance to become a mangaka is not measured by the ratings you get and trust me, there are many people out there who are trying for a long time (or at least they tell me so XD) and can't improve to your level. So the one last thing I have to write is:
GANBARE!!!! -^O^-