Intermittent heat/ac

Help! The heat and air in my '94 Prizm only intermittently work! Sometimes when I turn on my heat/air, it will work. Other times, for no apparent reason, it won't. I was once told that it might be a "relay." I've found then relays, but there are, like, three of them to choose from! Which one??

try changing the fan speed switch in the dash first that is what i had to do with my 90 prism

Thank, but I'm a doofus. Apparently the problem was that the radiator had never been flushed over the last 10 years. The accumulated sediment messed up the thermostat, which caused a sensor to shut off the ac/heat. Cleaned out the radiator and put in a new thermostat and it works fine. But thanks!