When I turn on the air conditioner the car really runs bad,but when you turn it back off it seems to run okay. I know it must be some kind of sensor gone bad but where do you start. Any help would be appreciated,thanks.

I'd begin by replacing the platinum spark plugs (ACDelco only), if they have not been replaced in the last 45,000 miles.

You may also have a dirty or sticking Idle Air Control (IAC) valve.
You can sometimes clean this yourself: you'll need a spray can of a good quality throttle plate cleaner and a soft bristle brush (I use a toothbrush but check it with the cleaner to make certain it doesn't melt from the cleaner) and some old rags.
With the engine warmed up, prop open the throttle blade and spray with cleaner and gently scrub with the brush. Spraying away the gunk and carbon build-up.
Once it's clean, start the engine (it will run rough) Let it run above idle speed and race the engine slightly until all the cleaner is burned out, then go for a road test. Your idle should stabilize and adjust itself when the A/C is cycled on.
I hope this helps.