I picked up sculpting a little over a year ago. The way I do it: I take an existing figure and sand down all details of the existing sculpt with a dremel tool. That provides me with a bare frame/skeleton.
Then I sculpt all details over that skeleton.
I use the following:
- sculpting material: I started out with green stuff, but now I use Aves Fixit Sculpt - a two-part component with a 4-hour curing time
- for sculpting: dental tools and picks, soft shapers, x-acto knife, anything you can use to sculpt, shape, cut, and add texture to your sculpting material
- for reference: any picture of the character I'm sculpting I can find online, or in books
I don't really have any other pointers, it's really all about practicing, practicing, practicing. It is a frustrating process too - no one becomes a master sculptor overnight. You can find a good deal of videos and tutorials on YouTube which may help you - just look for "sculpting tutorial", "sculpting clay", etc.
About sculpting new heads from scratch... your best bet would be YouTube again. It seems to be a lot of practice; I'm personally not good at it. I'd recommend starting from existing heads, and sculpting additional details over it (e.g. new hair, facial hair, new features, accessories) and once you feel confident enough, try to go for an all-new head.