My car squeals when the A/C is on. Why?

I have a 2004 Nissan Sentra SE-R, when I turn on the A/C and when I put it in drive and I'm at a stop light it squeals, sounds like it's comming from the belts. I try putting WD-40, but it only helps it a little bit then it starts to squeal again. I had my brother put it in drive with the brake applied, and it the belt was moving shaking and doing the squeal, it is only done when it's either in reverse with the brake applied or without pressing the gas or when in drive when the brake is applied and the gas is pressed. Can anyone tell me why is it doing that and what should I do. Thanks!