I'd like to learn

[Vipershark]

Kirby

Hoooooly crap, wow.

Okay, first off that canvas is gigantic. Resize your canvas to fit what you're showing before uploading. Don't post giant images with mostly empty space like that.

Also,

the circle tool makes really ugly circles most of the time.

You used the circle tool. It's really obvious because the circle tool makes terrible circles and you really shouldn't use it (or at the very least, you'll have to work on it to make the circle look better).

As Omnikariz said, you've got pixels of different resolutions on the sprite (the outline of the circle seems to be 20x20 while the rest of the sprite seems to be at 10x10 [also holy crap dude, your sprites don't need to be THAT big for showing. 4x scale is generally good enough in most cases]) so you'll need to put them all to the same scale.
Speaking of your outlines, you've got jaggies on the arms there and you'll need to get rid of those.

Also, your image host changed the image to a jpeg anyway so it's useless. Tinypic really isn't that good of a site to use for uploading images. Try using imgur or minus instead.

Finally, instead of making another new sprite, you should go back and work on your older ones before jumping to something else entirely.
You still have those other Yoshi and Paper Mario sprites that need fixing.