Hi,
if I get it right, your question is quite vast and does not seem to be exactly about the software package in itself, but rather about making yourself a background in population genetics... so, back to "square one", I'd say.
If I may suggest you a path, you may start by reading a population genetics textbook (e.g. P. Hedrick's "Genetics of populations"), then read literature about the coalescent (because this is what the business is about in fastsimcoal: applying the coalescent to infer population-genetic processes; you may start by reading the relevant chapters in Balding, Bishop, Cannings "Handbook of statistical genetics"), and then move to reading (at least) Arlequin's manual, which is in itself a course in population genetics... it will take time, but this way you'll know what you are doing with fastsimcoal, and why.
All the best,
Ivan