The first thing I think of is that the coolant level is too low, which could damage the engine. Be sure the coolant is up in the radiator. Check the radiator cold so you don't get hurt by hot coolant when you take the cap off.
The means of controlling the temperature of the heater air is that the temp control on the dash moves a blend door to regulate how much air passes through the heater core. You could check to see that the cable is attached at both ends and moves with the lever on the dash.
Here's a directory link for you: