1984 Dodge Truck heating issues

I have an old pickup that I got from my neighbor during the summer and now that winter is here its hard to see out my windshield and my truck is freezing because my heater is not working or is maybe plugged I want to dig in and see what the problem is but I don't know what im looking for and where everything is located. I tried finding some diagrams but couldn't find anything and I was wondering if someone could help me out and maybe give me some tips
thanks you