To elaborate on what Bikedued said: You want the leading edge of the brake pad to contact the rim ever-so-slightly before the trailing edge does so. This will eliminate squealing and shuddering. The ends of the calipers where the slots are located for installing the brake pads may have originally been bent slightly to achieve this. But since this bending occurred the brakes have been reversed from front to back and vice-versa. So now the calipers are bent in exactly the wrong direction. (As Whatwolf points out, this also caused the pads to oriented backwards so that they can slide out when the brakes are applied.)

The Park tool that Bikedued mentions is for re-aligning the calipers. If you are careful this can also be done with a very small crescent wrench clamped on to the flats of the calipers.
Brent