Determine the cause of the squeaking. Several things make bike brakes squeak: wet rims, dirty rims, new brake pads, or brake pads that are not adjusted correctly.
Try the easy fixes. If the rims are wet or dirty, clean them with a cloth. If your brake pads are new, you can file away the shiny surface or simply use them until they are broken in.
Check the angle of the brakes, which is known as the "toe in." The front or leading edge of the brake should be angled just slightly toward the rim. When you engage the brakes, this edge should hit first, followed by the rest of the brake pad. If only the leading edge hits the rim, your angle is too great.
Adjust the toe-in, if possible. Most new bikes have brakes that you can adjust by loosening the brake pads. If you have the bike owner's manual, consult it for the best way to adjust your brakes for that specific model.
File the brake pads to the correct angle if your bike does not have adjustable arms. You can also bend the brake arms to achieve this angle, but experts such as Jobst Brandt do not recommend this method.
Readjust the toe-in if the brakes start squeaking again. The toe-in will change as the brakes are used and become worn.