My advice is to specifically only use the pencil. The brush has a ton of anti-aliasing applied, which makes filling the image with color extremely hard and look really bad.
Hope this helps!
Anti-aliasing is when an art program will attempt to smooth out a line.
While this is normally a good thing, the paint bucket in paint isn't equipped to deal with this. Filling in a circle with anti-aliasing leaves these ugly white spots. because paint's paint bucket doesn't work with color tolerance, and rather fills the area of the EXACT color that you clicked on.