I have a 2 wire set up for my very old thermastat, just bought a new one at rona... one wire is black the other white... where do I install into the new unit... I tried what the book says but the furnace will not turn on, put the old one back on and no problems

Black white wire tells me that you have a 120, or 240v thermostat requirement. Are you sure you didn't buy a 24v themostat. Have to make sure the new thermostat is compatible with your system.

What type of heat do you have? Electric baseboard, forced air (gas or electric) , hot water(boiler).

Post brand and exact model number of old thermostat.
Post brand and exact model of new thermostat.