So you've determined that noise is indeed coming from the front brakes, where most late-model cars have their disc brakes.
Before you take your wheels apart, peek into the inspection opening in the brake caliper. Can you see the line of the friction material? If there's less than 1/4 inch of friction material, it's about time to replace the brake shoes. If there's less than 1/8 inch, you have no time to waste -- the lack of friction material could be dangerous. And it could be causing damage to the brake rotors, which can be more costly to fix.
If there is some damage to the brake rotors, you might want to have a mechanic smooth out (or "turn") any deep gouges in the discs. That process basically involves sanding away the scars to create a new, smooth surface. But if you've already done that, the discs might be too thin to keep using. That probably means it's time to replace them [source: Buckman].
Check your car's owners manual to find the minimum thickness for your brake rotors. Too-thin rotors are less effective at slowing you down -- and replacements probably won't break the bank.
You might also want to be prepared for the possibility of replacing your brake calipers. They're a hydraulic system, so they can develop leaks and corrosion under normal use. Since you'll be disassembling the brakes anyway, think about springing for replacement calipers.
Checking all these other parts may seem like overkill, but remember -- anything that adds unnecessary friction to your ride is reducing your fuel efficiency. And it might be compromising your safety as well.
Before you get started, take a look at the list of tools and parts on the next page. As a refresher course, you might also want to take a look at the parts and structure of a disc brake.