I have an older thermostat with 2 wires, one black and one white. On my new digital programmable thermostat I do not see a place to hook up the black wire. The istructions say to ignore wire colors and to use only the letter designations on the back of the thermostat to identify wire types. My old thermostat does not have letter designations on the back. My new thermostat has terminals lettered G, W, Y, R and Rc. Where do i connect the white and black wires?

in most 2 wire systems you have a hot (low voltage {24 volt} wire) that goes to R on your new tstat the other wire will go on W post to send message back to the furnace,,,,you can test at the furnace where the tstat wires are connected to determine which color is the hot wire