^^^ is right. Bleed your coolant system first. It's free, and it takes 15 minutes. Just run the car with the rad cap off and add coolant if the level goes down. Make sure that both hoses get warm, that's how you know everything is circulating. You could also have a massive air pocket in between your engine and your heater, causing next to no heat in your cabin, fatal to your engine in a short amount of time too. IF the level is fine and you are sure there is no air in the system, then change the thermostat. If that doesn't work, replace the temp sensor. If the problem precist check the continuity between your cluster and the sensor, maybe a faulty wire giving out a bad reading.