oh i love my ugly boy, so rough and tough don't care bout anything but me ♫
i'd suggest looking into shading of the human face. a quick google search for 'face shadow reference' should yield some helpful resources.
all in all, just keep observing, pick up artistic inspiration where you find it and practice! you're off to a good start!
i like it... the style reminds me of The Wild Thornberrys... good job.