Nice to see your initiative.
I advise you to blend the colors to get a smooth shading of forms,
then go for detail.
study anatomy while you are drawing, it will help you to design a
more convincing realistic piece.
Look how forms of the bodies look and try to achieve a similar result.
draw lines from confident moves, also smooth the lines in paint tool sai,
or either use linework layer.
Another tip is to draw by simplistic forms, you draw a stick figure showing an S curve spine, then draw boxes, ovals, cylinders to build up forms of arms, legs torso and head.
take a bit of practice of perspective, if you get a bit of it when used to forms, you learn it easily.
These are some of the fundamentals.
with this, you can build better drawings
the last piece shows a better anatomy and texture, I like it.