Hi San Weng
Its a nice start so far, your work makes fast progress. look at me, i’m still sketching 
To know what causes light and shadow, study the details. make yourself some study pages, my desk is filled with em. Work on the anatomical details, muscledefinition of the beast and master. also, the hands of the master could use a work out. I like the dynamic you added to him, just don’t overhaste your progress.
then, when nailed all details, work on the composition itself.
Composition:the heavy, dark forest need some light up, to help the feel for depht, i suggest (the rays of light are caused by the sun lighting up the athmoshpere) to make yourself clear what the background is going to be layered like. than, build up the fog layers. that could help.
for the colorscheme, thats pure matter of taste, but at the moment its a bit unballanced for my eyes.
Keep up it up, best luck and wish you loadsa fun
cheers