Basic rule of thumb is thus:
- adjust your saddle up and back until you are pretty close to KOP (do a search if you don't know what KOP is)
- pick a stem to allow you the amount of "lay out" you desire. Sporting riders like to stretch out more than cruisers. Let your back be your guide.
Racer types typically pick a stem that will allow their elbows to clear their knees while pedaling in the drops with a flat back. Tourist types often have "overlap" of their knees to elbows, but these folks rarely get low enough for this to matter.
Good luck.