That squealing sound when you start your car or rev the engine is a slipping fan belt. Besides being irritating, it could also cause more serious problems that will prevent you from driving the car until it is fixed. In most cars, the fan belt is connected to the alternator, which is mounted on a pivot. As long as the belt isn't worn out, you can adjust the tension by adjusting the position of the alternator.
Open the bonnet and inspect the fan belt. If you see signs of fraying or wear, take the car to a service centre and have it replaced.
Locate the nut holding the alternator to the curved adjustment arm. It is usually directly under the alternator or on the side nearest you. Find a socket that fits the nut and push it onto the end of a socket wrench. You may need to use an extension on the wrench to reach the nut.
Set the ratchet so that it engages when you push the handle counterclockwise and unscrew the nut until you are able to pivot the alternator. You may need to hold the end of the bolt on the other side of the adjustment arm with a wrench to prevent the bolt from turning with the nut.
Place a long metal bar on top of the alternator and push it in until the end engages with a solid part of the engine. Push down on the end of the rod to force the alternator outwards, tightening the fan belt.
Reverse the ratchet direction and hold the rod down while you tighten the adjustment nut. Torque it as tightly as you can, then remove the socket wrench and the rod. Test the tension on the belt. It shouldn't move more than half an inch when you pull up on it.
If the belt isn't worn, you have tightened it as much as you can and it still slips, try spraying belt dressing on the belt. It will help the belt gain traction and stop the squealing, but it isn't a permanent solution. Take the car in for servicing.
Fix a slipping belt as soon as possible. Slipping frays it, and it may break. Slipping also polishes the metal on the pulleys, which may cause a replacement belt to also slip.