hmmm, thats a big question, if youve got some anatomy/art experiance of drawing then it would make it easier.

I remember that I tried a head no long after I started and it was horrible! I left it for a while modleing "solid" objects getting used to topology and constructing clean meshes before attempting making a realistic head.

You need to understand topology nad edge loops etc as well as having a clean, organised mesh, not to mention UV mapping and texturing when it comes to that. As it helps if you wanted to animate it. Theres lots of help on the net with regards to topology (have a look in the WIP thread theres a topology thread there) and tutorials on modelign a head.