First, check to see that both heater hoses are getting warm the same amount If yes, air blend door may be not operating correctly due to vacuum leak or bad actuator. Failling check valve near vacuum reservior can do this too.

Check coolant levels. If not full then fill it up. Second do as rustfarmer said. If you still arent getting heat then it sounds like your heater core is clogged. If so have system flushed and new coolant added. One other issue maybe that your heat control knob is stripped or inner knob plastic handle is cracked. Pull off knob and check the back of knob for cracks. Good luck and keep me posted.