Nest Thermostat Converting from Line Voltage to Low voltage

I have two thermostats in the first floor of my house, one for heat and one for a/c. The one for the a/c is programmable, and low voltage ( or so I have been told) the one for the heat is line voltage. I would like to hook the heat up to the programmable thermostat. To make one thermostat. What do I have to do to convert the line voltage to low voltage? Someone told me about a relay, of which I have no clue, I also saw an adapter in Home Depot, for what seems like just this type of thing. Is it that easy or should the relay be installed, or both?