Once you have subdivided your mesh you will need to clean it up a little. You might have created more points than necessary and we want to keep our points to a minimum so that it will be easier to edit further up the road.
To do this you must first select all the verticies within your silhouette. Leave the border edges alone for now they are still needed to define the shape of your object. Also slelect the inside verts of the neck border leaving the outer 2 alone.
Then press the delete key on your keyboard. This will clean up any hanging verts leaving only the ones that exist at intersections.
Once your mesh has been cleaned up, again select all the inside verts (including the ones on the inside of the neck)
Next thing we want to do is bring this model into the third dimension. We do this by simply going to the front viewport and pulling the selected points toward the the outer edge of your drawing.
To give you more of a sense of form while modeling, it is a good idea to create an instance of your object. An instance is an object that shares the same shape node but has a different transform node. Making it an exact duplicate of your original geometry just at a different point in space. Therefore, any changes you make on one side will take place on the other instantaniously.
To create your instance, go to edit>duplicate and open your options dialog box. Check -1 on the scale X direction, then under geometry type check instance.
Now you have the beginnings of your head... not much to look at now but it soon will be.
When modeling, I find it easier to see my wireframe while I'm working. To view your wireframe in your viewport go to shading>shade options>wireframe on shaded.
To start shaping your object, start grabbing verts and begin moving them only in the X direction. Use your front view as a guide. For example...go to your side viewport and grab the point that defines the corner of your mouth and then move it in the X direction to where the corner of the mouth is in the front viewport. Don't worry if your mesh looks really rough at this stage, that's the way it's supposed to be, we're just roughing out the form of the head at this point. Think of this process as if you were carving from stone. First you start out really rough to get the form of your object, then you go in and define the forms a little better... and then (but no sooner) you go in and create details.
Next look at the point where your eye is in your side viewport, make sure that it is directly in the center of your eye in the front viewport. Then create a diamond shape with your split polygon tool. This will begin the radial of your eye socket.
Then subdivide the diamond one step further so that you may better define the socket of the eye. As you split your edged try and split as close to the center as possible.
To visualize the eye better, create a nurbs sphere and place it in world space according to your front and side viewports. This doesn't have to be your final eye... just a place holder so that we can form the eyelids around it correctly.