So I have 1 80mm fan in my case (which has 2 places to mount it: one on the side and back), so I am wondering where should I mount the fan? And should it be intake or exhaust?
depends. You want to remove all the heat to outside of the case so make it as exhaust. you can buy more fans if needed. there should be 1-3 intakes at the front and one as exhaust.