Hey man welcome to cubebrush.
Now for this piece here, I recommend using reference when trying the light and shade the face.
Another thing I notice is that there isn't really any defined forms. Drawing the face then painting it will help with this.
You have good colors here but the problem lies in understanding of the forms and the form of the human face.. Take it slow an start simple. A line art then colors. Do more studies to get a better understanding of the value shifts and form of things.
Keep up the good work man and remember the have fun.