Squealing can be reduced by "toe-in" - making the front part of the brake pad touch first.
Check out Park Tool's instructions on servicing side-pull brakes: http://www.parktool.com/blog/repair-help/sidepull-brake-service
Squealing is the result of a harmonic resonance from a slip-and-stick phenomenon, similar to how a violin bow resonates on a string. The brake pads grab the rim and are pulled forward by the force of the wheel. The arms must at sometimes jerk backwards, but if you are still braking, the arms are flexed forward again. This is repeated again and again many times per second, and this creates the squeal. Because of this, flexible and less expensive arms tend to squeal more than stiff calipers of better brakes. It is possible to reduce squeal by having the leading edge of the brake pad strike first. This tends to reduce the back and forth jerking of the arms.
But before you go trying to toe-in, make sure you're working with parts in good shape:
Examine your brake pads. If they are old and hard, worn deeply, or full of junk, replace them. Otherwise, use coarse sandpaper to take off the surface to bring out fresh rubber. You only need to take off a tiny bit.
Clean your rims. Road grime, bike lubes, and brake material all collect on your rims. Rubbing alcohol, vinegar, Simple Green, a product marketed as "degreaser" can all work. Don't let any get on the drivetrain or in the hubs. Also, you'll need rags.
True your wheels. There's no point in trying to adjust brakes on a wheel that isn't true (round and dished is good, too). And you can't true a wheel if the hubs aren't in good shape.