I'm a first-time spriter, and need criticism. Here's a sprite sheet of mine.

[Vipershark]

I guess it kind of depends on what you're going for here.
You have a bunch of (completely unlabeled so I have to guess what you're trying to represent) poses that are largely all edits of the exact same pose.
Your walking frames are essentially just shortening and lengthening the character's legs and while I suppose that *can* work depending on the style you're going for.... I don't actually have any idea what you're going for.
Is this intended to be an RPG type style where you'll add side and back poses as well? Or is it some kind of tamagotchi-type thing where he always faces one direction?

Regardless, the tiny sprite size doesn't really help because there's... really not much you can do with so little area to work with.
I can say though that on many of the poses the arms seem to be coming out of his lower abdomen as opposed to his shoulders, so you probably want to move them a pixel or two up.

In addition there's the whole lack of shading and whatnot, but... again, this depends on the style you're going for.