If your drawing a face and get to the mouth region and don't know where to go from there then watch this video to see how you can shade in lips. Fist you'll draw the basic lip outline. Depending on who you're drawing the lips might be fuller or thinner. The tricky part it the shading. The shading is to show depth in the lips. You'll want to give a shine in the middle to greater depth. Next you'll want to try drawing a partially open mouth.