no heat

My 2000 blazer has no heat. When I turn on the heater/defrost it only blows cold air. Could this be the thermostat? I wanted to flush my radiator fluid also, what is the proper way to do this and where are the drain plugs located. I should use dex-cool right. What is the proper way to mix 50'50 dump half out of jug and fill half up with water in the jug? If not the thermostat what else could be the problem/ What is proper way to replace thermostat and where is it located exactly?? Thanks, starting to get cold here in PA...