2002 Dodge Ram Heater blows cold air

HI

We own a 2002 Quad Cab Dodge Ram 4.7l engine. When the vehicle is just sitting and idleing it will only blow cold air and then when you start driving it will heat up and blow hot air. I think we just have to replace the thermostat but is that a huge job and could it be something else?

Thanks for your help.