To start prolog try
	prolog
	-or-
	prolog -f foo.pl

To load a file try the below.  NOTE:  Don't use the extension ".pl" when using consult.
	consult(family_prolog).

To ask a question try
	parent(X, Y).
	parent(X, mary).

Semi-colon means "show me more answers".
Return means "That's enough".

Capital leters are free variables.
Lower case words are constants.