Car overheating

I have been having some issues with my car and the coolant reservoir, I believe. So when I am driving my car is fine, but if I am sitting in traffic for too long with my car running, after a while my car will start to smoke white smoke. I look and see that the coolant is gone. I have used water in the mean time but from calculating my drive from home to work which is about forty five minutes today, after filling the reservoir before leaving home, once I got to work it was half full. I am so confused as to what the problem is. Of course once my car starts to smoke the thermostat shows "H". Please help with some information I can use to find out something to do. I am not sure if it is a leak or what. I did come across the product Blue Devil Radiator and Block Sealer. I am not sure if that would be for my situation or not but if able to give info on it I really would appreciate it.