Start with the simple and check your radiator fluid, if your fluid is very low. You will not get any heat at all though your heater core and will end up with a blown motor.
Once the level has been checked and is full, check the temperature of your heater hoses. The inlet should be hot and the return slightly colder, if there warm and about the same temperature. Good chance your thermostat is stuck open and your coolant is cycling through your radiator more then your heater core.