Pillow shading. Find a light source, it's brighter where that light is and there's shade where the light can't reach. If you're having trouble then draw a little light beam in the sketch then color the shading, you can erase the make-shift light beam afterward.
Her neck is just off.
Finally, her legs. They wouldn't go straight to the side, they'd go down a little. Try looking at other pictures to get the anatomy right or even look at yourself in the mirror doing that pose. It helps a lot.
great for a beginner.