I am trying to install the honeywell programmable t-stat rth230b. The problem is my old two-wire thermostat has red and green wires . Where do the hook-up on the new one? Thanks

Did you get a set of instructions with new tstat?
Go to the furnace ,look and see what the stat wires on that end are connected to. I`m guessing with only 2 wires you have a heat only system.in most cases you would hook the red to r/h and the other wire to W post,but check it out at furnace first