That Spidey drawing is actually pretty good. The colouring is messy but just great, although I don't need to learn a new computer program to get such results. Some poster paint, a brush and some black ink should do the trick just as well. Just my two cents, but I like doing stuff the old-fashioned way.
You're pretty talented for a 13 year old, though, that's for sure. Keep this up for a few more years and you will be a pro.
One thing though: all in all, it's not very hard to draw people. Can you do backgrounds as well, I mean, can you put those people you draw in a "realistic" setting? That's the really hard stuff.
Nevertheless: keep posting your work. I'm intrigued.
@ Melchior: damn, I thought you were Dove. Damned avatars...