A car heater that isn't working could be caused by several simple issues. Either the blower isn't working, in which case you may get warm air but not much of it, or it is blowing air but it may not be very hot. Here is what to check if your heater or defroster isn't working:
Coolant/Air lock - Did you top off the coolant as described above? Low coolant may prevent any from circulating through the heater core, as can an air bubble in the system. Make sure the system is full and free from air by filling it with the engine running and the heater controls set to maximum heat.
Thermostat - If the thermostat is jammed open causing the car to overcool, there may not be sufficient heat in the coolant to provide hot air. Test, or replace the thermostat, as mentioned above, if you suspect this to be the cause.
Heater Control Valve - When you set the heater temperature dial to hot, air being passed over the hot heater core should be ducted into the cabin. If it isn't, it could be because the heater control valve is not sending hot coolant to into the system. The heater control valve is typically not service able and is just replaced if faulty.
Blend Door - On older cars and many simpler systems, a physical flap directs air from the heater core or outside into the ducts inside the car. This door can become stuck or disconnected from the control slider or knob. This part is typically serviceable, but not particularly easy to get to.
Blockage - The heater core is made up of many very thin tubes of metal, and if it becomes blocked the coolant may circulate as normal around the engine, but not flow through the heater core. Typically you can't fully cure this problem once it gets really bad, but you can prevent it with periodic flushes of the cooling system about every 30,000 miles or two years.