Overheating problem

When I am parked idling my van starts to overheating In about fifteen minutes after the van is warm unless I have the heater running. I am not the best mechanic but I have a few friends that can help me if I cannot figure it out. I just cannot afford to be replacing things that are not the issue. I am out of work and the van is my lifeline. I am not sure where to start with it, I am hoping for some tips or ideas on diagnosing the problem without spending much money (that I do not have I am broke pretty much). So hopefully I can fix it at home. I just got the van and am not familiar with it. I do not know where to start, it has not overheated while driving only when I have been parked with it running and the heater not on. I have been told maybe thermostat or water pump. It does have that annoying belt squeal that comes and goes. I was told that it could be the water pump pulley squealing, but I have also been told it could be as simple as a sensor. The fact that it does not seem to overheat when I have the heater on is what is confusing, only when it is off have I had the problem. So I have just kept the heater on medium constantly when it is running. That is not going to be a very good solution when it turns to summer. I do not want to have the heater running in my van on a hot day to keep from overheating. I know I have not been very descriptive but I just got the van. I have only had it happen a hand full of times before I noticed it only happens with the heater off. I would like to fix it not just bandage it running the heater. Where do I start? I am far from a mechanic so in language I can understand please help me with any suggestions or ideas. Thank guys.