1995 Chevy Cavalier Low engine temp, no heat.

I have a 1995 Chevy Cavalier, and the issue is with low engine temp, and no heat. If I crank up the car and let it sit for about 30 min, it gets in normal operating temp range, and the heater works ok. When I start to drive the car, the temp drops down to nothing and the heater go completely cold, just like the car was not even running. The colder the outside temperature is, the lower the car temperature gets. On a warm day, it operates at higher temperatures, and the heater gets warmer, but still not hot. What would cause this to happen. I heard that it could be the thermostat, but I don't know. Its not leaking any coolant anywhere. I thought that if it was the water pump, that it would be different, and run hot, too hot.I would hate to start replacing parts and it be just something simple. Thanks for the assistance.