First check your coolant level. If that is good, either your thermostat is stuck open, or you have a problem with the heater controls. Have someone turn it to hot and cold repeatedly with the key on engine off. Stand on the passenger side where the heater core is. There is a valve that controls the amount of hot coolant flow into your heater core. If nothing happens there, then you need to check if the valve is bad, or if you have a bad circuit or switch in the dash. To check the thermostat there are two ways. 1 take it out, hang it by a wire and submerge it in hot water. If operational you will see it open up. 2 start up the engine, grab the upper radiator hose and squeeze it periodically as the truck warms up to operating temp. the thermostat shuold pop open and releive the pressure in the hose, (please be careful, it will be hot). If the hose never builds up pressure from the time of starting it when the engine is cold to it's operating temp. your thermostat is bad. If you haven't a clue, take it to a reputable shop, but dont let them sell you everything under the sun. And one more thing i forgot to mention is the possiblity of a plugged heater core.
