Firstly ensure each brake pad lines up with the rim by pushing each brake arm towards the rim. If needed, undo the nut on the rear of the brake shoe, line the brake shoe up with the rim and tighten the nut.
Next check that the brakes are centred evenly on the rim, squeeze the brake lever and release. If one side of the brakes stays in contact with the rim you can increase the spring tension in that side by turning the spring adjustment screw clockwise.
Next step, if needed, adjust the brake pad to rim gap by adjusting the brake barrel adjuster where the brake cable enters the brake lever. Screwing the adjuster into the lever will release cable allowing the brake pads to move away from the rims, screwing the adjuster out will bring the pads closer to the rim.
Finally check the brake operation. Pull on the brakes and make sure they are tight and can lock the wheels up, this can be easily done standing at the side of the bike and pushing bike forward while applying the brakes.