Scion TC, AC is acting up?

for about two weeks now my AC sometimes works and other times it doesnt. i've recently noticed that if i leave the air conditioner on before i turn off the car then the next time i go and turn my car on, it works just fine. but when i turn the AC off and then the turn off the car, when i go back and turn on my car nothing happens. i hear the clicking sound like the its turning on, but no air is being thrown out. i know i should probably take in to a mechanic, but i want to at least know what the mechanic is talking about before i take to them.