Could be lots of things. Is this a new development or has it always done this?
Could be low refrigerant charge, could be a worn compressor, could be a bad electric fan, could be a bad mechanical fan clutch.
Is your electric fan running when you turn on the A/C? If so, then it isn't the electric fan. What year is the car? If this is an originally R-12 car, then has the car been retrofitted to R-134a. If it has, then that can explain some inefficiencies of the A/C. How many miles on the car? Is it the original A/C compressor? Has the fan clutch ever been replaced? Is the engine running hotter than midpoint on the temperature gauge when idling but then cools down when driving?
We need more info to help you out!