There should be a knurled barrel adjuster on the brake lever. Unscrew it some and your brakes should "tighten." When you have 'em where you want 'em, screw the lock ring (the big round thing on that barrel adjuster) back down against the brake lever, locking it in position.
If you can't get it tightened enough that way, you'll need to take slack out of the cable at the brake itself. Loosen the nut that holds the end of the cable in place and pull the cable tight, then tighten the nut back up. You can then fine tune using the barrel adjuster on the brake lever.
Answers & Comments
There should be a knurled barrel adjuster on the brake lever. Unscrew it some and your brakes should "tighten." When you have 'em where you want 'em, screw the lock ring (the big round thing on that barrel adjuster) back down against the brake lever, locking it in position.
If you can't get it tightened enough that way, you'll need to take slack out of the cable at the brake itself. Loosen the nut that holds the end of the cable in place and pull the cable tight, then tighten the nut back up. You can then fine tune using the barrel adjuster on the brake lever.