Is the engine running at a normal opperating temp? IF it is, with the engine running, hot, and the heater on, feel both heater core hoses. THey should both be hot. If one is hot and one cold, chances are the heater core is plugged. If both are hot, then the blend door needs to be cheked. It determines if you get heat or AC.
Let me know what you find or if you have other questions.