I will put what to do in words
as well as I can.
Remove one of the front wheels. Turn the part that the lug nuts screws into (this is the "hub" - the larger diameter "shiny" rotating piece is the brake rotor). While looking down on the edge of the rotor look for the parts that come closet to it. The parts closest to the brake disc are the brake pads and you should see one on both sides. The part holding the pads are the brake caliper.
Now, this is where the description gets hard based on if the pads are worn out or not. You are going to have to look rear close at the pad and the metal surface that the pad is attached to. The pad left on the metal surface is what wears down. When it is worn all the way down to metal you have used them too long and have more than likely damaged the rotor. There is a little slot cut through the pad. When the slot is no longer there it is time to replace them.