As the pads wear you need to adjust the brakes to make up for it. As you look at the front brake from the saddle the left hand pad is moved by the brake lever, this one is adjusted in various places along the cable. The brake pad to the right is adjusted using a hex key through the spokes, if you operate the brake and see the disk bending off to the right then put the key in and tighten it up one click (or more as you get used to doing it). Check that the wheel rotates freely, then repeat until it doesn't and undo the adjuster one click. You might need to squeeze the brakes after you back it off before testing again. When you reach the end of the adjustment it seems to behave strangely and not adjust the same, this is when it's worn and needs replacing.
Edit: Don't try and spin the wheel with the hex key in place.
