Well ltutorial, you have a very nice start there. You're applying the basic shapes very well.
I would like to see something with more complex design coming for you, like an ornamented armor. It will help your design sense for more complex shapes
AA is the technique used to soften the serrated effect of the object line, there are people who do not use this technique in the shadow, it is the same idea of the black lines but in the shadow, as in the example of the yellow part.
at the time of creating the sprite remember to always start with a black background, so that you learn to use the AA without seeing the outline, and learn to do the AA in the shade to smooth the color degradation
Yellow - light colors green - medium colors Blue - dark colors red - reflection