Okay, for a 12 year old that can't draw felines (apparently), this is fairly good.
I'm not sure if you were aiming for this or not, but the lines where you starting drawing the mane is blurring the black lines. If you want this effect, good, it's working out nicely.
If not, I suggest you put everything on separate layers, to make it all nice and neat.
I would also like to commend you on the pose and the shrubbery things on the bottom, but also would like to point out that the entire picture would look.. cleaner? if you used a clear, bold, and defined pen tool like... a pencil or a pen on SAI. I think this effect comes from the paintbrush tool, thus smudging everything and stuff.
Your eyes are really nicely done, but again, would be incredible without the smudging. Large hairs could be grouped together as one entire outline instead of manually drawing each and every hair.
What really irks me is the tail...
Sorry, but I'm a bit of a tail freak XD
Suggestion: practice drawing loose, flowing lines for tails and hair, since most of these aren't stiff and kinked.
Again, you're 12 and this is pretty good artwork for a 12 year old C:
Keep practicing your art, and I'm sure you'll be as good as the other artists on Res! Keep in mind, they're probably so much older than you.