How To Begin
Now that your designs and references are prepared, it's time to take the first step. Where to start? Different people have their own preferences. Some start from the eye or mouth, assembling the pieces into a full head. Others build the entire body as a rough shape first, then hack it into shape by slicing and adding detail. There is no universal "standard" or "best" way, so find your own through experimentation. For the beginner, it is probably easier to treat the head and body as separate parts to be joined later. In this way, you can focus on just the face or the body structure. But always bear in mind how the parts will be joined, as well as their relative proportions.

Head Over Heels
The easiest way to make a head for the novice is probably to add a sphere. From there proceed to modify the shape into a more head-like form. If you can't model facial details yet, simply apply a texture map! My first character models were all done using this method. As you progress in modeling skill, try to create the eyes, mouth... etc, using actual geometry.

Here are some images depicting the steps I took to create a "realistic" head of Angelina Jolie as Lara Croft. I started with some scanned photos found off the internet, which I cropped and sized to fit two reference planes forming a cross along the vertical axis. Starting from the front view, I built the facial features point-by-point and poly-by-poly. I alternated between to the front and side views to create the profile, adding depth to the flat face outline I started with.

As you can seen, this method allows good facial detail but it requires patience and a good understanding of the human head in 3D space. There are many tutorials available on-line that goes in-depth into the various methods of head modeling. Check them out:
Head modeling by Veli-Antti Rautiola
Eye research by Graham McKenna
Modeling an Ear by Todd Grimes

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