Okay, good first attempt. With faces, the shadows dont just start at the outside as dark, then get lighter as you move around as a sphere does. Look at some reference pics to get an idea for where different shadows go to define the nose, jaw and chin bones, and folds in the skin that everyone has. The lack of actual defnition of these things makes the face look a little flat. The opacity tool is very handy, if you look in the bottom right you can see 2 sliders, one of which controls opacity. the max is 255, which is pure opaque. Turn it down a bit and it will make it more transparent. Going over it again makes it slightly darker. Very effective and easy way of building up tone. I like the position she is in, and the proportions look about right. Maybe change the chin so it isnt so slanted? Get some pics of women's heads from different angles, and look at how the face is built up, this will help a lot! And also, the blend tool with opacity will be good. Hope I've been of some help!