When I start my car, it'll run fine, but whenever I turn on my air conditioner my car will just turn off. Why??

Your throttle body prolly needs to be cleaned. I've seen it happen several times. The a/c places an extra load on the engine and the vehicle compensates by opening the throttle a little more to allow more air into the engine. When the throttle body gets dirty, it doesn't allow enough air into the engine. I would take the throttle body off to clean it, but you don't have to. You can just remove the air intake assy., unplug the throttle body connector, and wipe the inside with a rag that's damp with carb cleaner. The throttle plate is spring loaded, but you can rotate it open and stick a screwdriver handle inside to hold it open. There will be a layer of black carbon/soot inside that you want to clean out. The carb cleaner will help dissolve it. Wipe away as much of the black as you can.