Not bad for a start.
It was a bit generic but surprisingly well thought out. It wasn't great, but everyone sucks a bit while they figure things out. Trust me, mine was terrible. Here's what I would improve.
1. Frame size. 400 by 550 is the standard, you should try to keep it there.
2. Backgrounds. You had some painful jumps, try to keep it looped in a way no one notices.
3. Teleporting. I have no idea why everyone thinks Sonic can teleport and fly. Or just ignore gravity, as the case may be. I mean, c'mon.
Still, it's a good start to your spriting career. Good luck, kid, you're gonna need it.