Adding detail to your drawing consists of using smaller lines to pick out certain parts of the face. You can add eyes, a nose and lips. Throw in where hair and eyebrows would be. Remember to keep it simple and not over do it.
You can also erase lines that you no longer need. Some lines may now look out of place. Draw a new line first and then erase the old one. You'll see it "move" better.
From this starting point, you kinda need to decide what style you want. You see how we can easily make this a cartoon, but for this tutorial we'll go all out.