Check first your coolant level and refill if low. Also check the operating temperature of your engine coolant and if your coolant temperature is below specification you should try to check the condition of your thermostat (it might be stuck or damaged).

If your coolant level is still in specification and your thermostat is still functioning well. Try to check the heater control valve operation. Check the heater control valve cable or vacuum hose for proper installation. Move the heater temperature control from hot to cold several times and verify the operation of the heater control valve. With the engine at normal operating temperature and the heater temperature control in the full hot position, carefully feel the heater hose going into and exiting the control valve. If one heater hose is hot and the other is much cooler, replace the control valve.