"Don't I have to go to PC A and say which folder(s) it will allow access too then then to PC B and tell it somehow to go and establish access to those folders."
Broadly, Yes. After allowing sharing on the target folders (on A) go to B and find My Network Places and Add Network Place.
I would turn off firewalls on both machines while setting up.
If possible I would connect everything by ethernet while setting up to avoid the vagaries of wireless.