Why is the heat in my truck only blowing cold air?

Any nunber of things could be wrong: Coolant low, heater core clogged, thermostat stuck open, etc.
First, look to see if anything is leaking: radiator, hoses, engine. Then, check to see if the heater hoses (both!) are getting warm. The thermostat is harder to diagnose. Do you have temp gauge or just a light?
Let us know...

BTW: what are you driving?