Does the car's engine get to operating temp? If not, thermostat is the place to start. If so, have you checked the heater control valve? This valve is what controls hot coolant flow to the heater core. It may not be opening or is clogged not allowing coolant to flow to the core. It is located at the firewall and you can trace it down by following the coolant hoses from the block to the firewall. Feeling them will tell yo if there is coolant flow (BOTH should be hot, if only one is hot then there is no flow to/thru the core) and if it is hot enough.
You can also try putting your hand on the heater plenum and see if you can feel it getting warm.
I had the same problem out of my 94, went to auto zone and got a "flow restricter" for the exiting hose of the heater core ($11.00) and while i had the heater core lines off i flushed it using a garden hose (ran the water through both hoses till the water was clear) this worked great,the car has awsome heat now. hope it helps you.