Here's a tip or two that may get you started in the right direction...

Go to your local library and find some anatomy books that shows muscle form and structure... and then study how the muscles flow and the direction....

Now, with that in mind, if you have done any traditional art ie drawing sculpting etc, you already have a good fundamentals on 3d modelling - the only difference is the modelling you want to do is on the computer.

There is no real area to start with, its mostly preference, but a good idea is to start with the body and extend to other parts of the body if you aren't using reference images on image planes...this way with the size of the body, you'll have a decent idea on how big to make the head.

The big picture (or many) in this case is reference material.. reference, reference, reference....its what that boils down to.