It could very easily be brake dust. Try cleaning the rotor and caliper area and see if that makes a difference. In some cases the pads can get glazed from heavy use, which can cause squeaking. To check for glazing, you would need to remove the caliper and look at the pads. A fine sandpaper can be used to slightly rough up the pad surface if needed. It all depends on what you are comfortable performing on the bike. These two things are very easy, but if you were experiencing other symptoms, like vibration when you brake, then there would be more to it then dust or glazing. Try the cleaning first and go from there. A high pressure water hose often will do the trick.