There are quite a few things that might be causing this problem. The first thing you should do is take the belt off and start the car just for a few seconds to see if the noise is still there. This will verify that the sounds is coming from the belts or pulleys.
To see if it is the belt that is causing the noise you can spray some water on the underside of the belt where it contacts the pulleys and see if that quiets it.
To see if it is the power steering pump you can try to steer it left and then right and see if the noise changes. If it does then it might be the PS pump.
If none of these are the problem then you might need to check the alternator. Sometimes they start wearing out and cause this type of noise.