Hi David
there are quite a few very experienced sculptors over on Historicus Forma who would be able to give you more advice.
from my point of view, i would start easy by first re-doing arms or legs on standard figure, just to get some practise.
i would also pick up the following books, which have all been a big help to me:-
1 - Conversion and scratch building of model figures by Andrea press
2 - sculpting and body language by euro modelismo
3 - Dynamic wrinkles and drapery by Burne hogarth
all are excellent in helping you learn to sculpt.
i also use the 1/35 Academy figures by FRIULMODELL as these are basically white metal frames that you can bend to shape and then sculpt over. I generally use Magic sculpt a it is easy to use and shape
Try
michtoy for a lot of the products and books
HTH
Keith Forsyth