Mike is dead on Right. It in all likelyhood is a leaking heater core. Here is what you can do as a temporary fix. Go to AutoZone, Checkers, or your favorite parts store and buy a heater hose adapter and two clamps for the heater hose. The Adapter is a small plastic or metal piece that will fit the two different sizes of heater hose on your truck. Disconnect both heater hoses at the firewall and "Jumper" the two hoses together with the Adapter and clamps. Check your coolant level in the Radiator and in the overflow jug and you will be good to drive until you can get the heater core fixed. This will not harm anything and will only cost a few dollars for the adapter, and besides, you probably won't be needing the heater for a few months.
Here is more Cooling system info:
http://www.automotiveforums.com/vbul...+system+advice