Audiophiles - I need help with home audio

I am in the process of finishing my basement and need some recommendations, or rather complete help in picking speakers and a sub that will work with a stereo receiver I acquired to complete a home theater system. The receiver is a Harman Kardon HK3490
https://www.crutchfield.com/S-R7838...-Kardon-HK-3490.html#Inputs and Outputs Notes
that I received for free. It appears to be a solid unit and I would like to use it, but I have no idea how to spec the speakers. I have four locations (two front, two rear), on the wall where I would like to mount bookshelf-type speakers to the appropriate mounts. The locations have the standard black/red wires running to them. I would also like to find a sub that will work with the receiver. I'm not sure what an appropriate price is.. $70/speaker? Is a center channel needed?

The input from my TV will be an optical audio cable. So, at least I have that figured out...

Any help?