I have a 2000 dodge ram 1500 4x4 with not even 50,000 miles on it. Lately when I've been driving with my heat on, the heater will start blowing cold air then my temperature gauge says my truck is overheating. What is causing this? I haven't tried replacing anything yet. What should I start with?

Check your coolant level to see if it is low
Check you fan accessory belt to make sure it tight and now slipping.
Your water pump could also be going bad. You can take off your radiator cap when the engine is cold and then start an run your vehicle till it gets warmed up enough to open the thermostat. You should be able to see the coolant flowing from the top return hose.

Coolant level low. Top it up and see where it is leaking.It blows cold air because no coolant is getting to the heater core.