If there is too much play in the brake lever when applying the brake then the brake cable needs to be tightened.
For minor adjustments use the barrel adjusted, turn anti-clockwise.
For great adjustments release the locking nut, move lever forward, tighten nut. Continue to adjust until there is less play in lever while making sure there is no rubbing of the pads on the rotor.
If there is rubbing from the brake pads when the wheels are spun then this may be caused by a bent rotor or by the caliper not being positioned parallel to the rotor.
By spinning the wheel and observing the rotor you'll be able to see if it's spinning true. If it's bent then it needs to be replaced.
If the rotor spins true and there is rubbing then release the mounting bolts a little, and by observing the gap between the rotor and the pads careful move the caliper so the rotor is positioned in the centre of the gap. Tighten the caliper and check for rubbing, you may need to attempt this process a few times as the movement required is very small.