How to Adjust Bike Brakes

Bicycle brakes become less effective over time. Cables stretch and pads slowly wear away. If you can squeeze the lever fully on without the brake engaging, urgent action is required.
Test your brakes
Stand next to your bike. Squeeze the front brake lever and try to push the bike forward. The rear wheel should lift and the lever shouldn't touch the handlebar. Now repeat this test with the rear brake. The rear wheel should lock and skid as you push the bike forward. If either brake isn't working properly, it's likely to be a result of slack in the cable – unless your bike has hydraulic brakes, in which case they probably need 'bleeding' to remove air bubbles. (That's a job for the bike shop or a confident home mechanic.)

Visual checks
Is the brake properly set up? V-brakes have a quick release so that the wheel can be removed and refitted easily. The 'noodle', which is a J-shaped metal guide tube, can be disengaged from its cradle. In this state, the brake will not work. Sidepull brakes often have a small quick release lever on the caliper, enabling it to open wider (as shown). Ensure this lever is closed, otherwise the brake pads will be too far from the rim.

Examine the brake pads. There should be a good thickness of braking surface remaining. (If your brakes make grinding noises in use, there's no pad material left: you're applying metal to metal!) Time for new pads? Head to the bike shop.
Adjust cable tension
All cable-operated brakes should have a barrel adjuster – a hollow knurled bolt where the cable exits the lever or enters the caliper. Some bikes use 'inline' adjusters part way along the cable outer instead.

to increase cable tension, turn the barrel adjuster anti-clockwise. Try one full turn initially, then half turns, repeating the brake test periodically. If the barrel adjuster has a threaded lockring or locknut, unscrew this to enable the barrel to turn, then screw it flush to the lever or caliper to keep the barrel firmly in its new position.
Re-clamp cable
If a few turns of the barrel adjuster don't solve the cable tension problem, try re-clamping the cable. First wind the barrel adjuster back in. Then undo the bolt that anchors the cable to the brake.
Use one hand to squeeze the brake mechanism together. This is easy with sidepull brakes and V-brakes: simply hold the brake blocks against the rim. With a cable disc brake, push the caliper's brake arm up to engage the brake. The brake doesn't need to be jammed on; just touching the rim or rotor is okay. Don't let go of the brake until you've re-clamped the cable.
Then with your other hand, pull more cable through the cable clamp, until the cable is just taut. Let go of the cable now and tighten the clamp bolt.