Can't get fan working on thermostat
Connect the green wire to the G terminal in the furnace, and the other end of that wire to the G terminal on the thermostat. Connect one of the black wires to the W terminal in the furnace, with the other end of the wire connected to the W terminal on the thermostat. Then connect one of the red wires to the R terminal in the furnace, and the other end to the Rh terminal on the thermostat. If it's a fancy WiFi thermostat, you could connect the other red wire to the COM terminal in the furnace, with the other end of the wire to the C terminal on the thermostat

If you ever add A/C, you'll connect a jumper between Rh and Rc at the thermostat. Then you'll run a wire from the Y terminal in the furnace, to the Y wire on the thermostat