Click on the resize button on the left side of Piske. 64 by 64 is a pretty good size. 32 by 32 will be harder to draw in, but give a more "pixel" game style when you bring it into Scratch. When you're done, click "resize."
Find the black square in the bottom left corner of Piskel. Click on that to open the color chooser. The giant square in the middle is how bright your color will be - click to move the white circle into the bright parts. The rainbow on the right is what your color will be - click to choose a different hue.
Find the little onion on the right side of the screen, and make sure it's yellow. "Onion skinning" is when you can see the old frame behind your new one - imagine you put tracing paper on top of your first frame, and will draw your new picture on the tracing paper. It helps you figure out where you drew things in the last picture.