Your brakes have to be in good repair. Failure to be able to stop can be life-threatening not only to yourself but to others. You can also damage your expensive bike and maybe someone else's gear, too. There are three elements to bike brake repair: inspection, pad replacement and adjustment. Inspect your brakes before any ride.
Inspect the brakes. Check for brake-lever tightness. Check the pads. There should be 1mm to 2mm clearance between pad and rim when the brakes aren't applied. When applied, the brake pad should be centered with the rim. Check for over-worn pads. You should be able to see grooves and a pattern.
Lubricate the brake lever with a couple drops of oil.
Replace the brake pads. Using an allen key, unscrew the brake shoe from the calipers and replace the brake shoe. Replace the washers in the order they were removed. Adjust the brake pad position by wiggling the shoe before tightening. The shoe must be parallel to the rim and in the middle. The shoe must not be allowed to snare the spokes or touch the tire. The leading edge of the pad should make first contact; adjust it using the wobbly looking washer. Moving it around will change the angle.
Make your final adjustments. On sidepull calipers, minor adjustments can be made with the cable adjusting barrel. It's the barrel at the very top of the calipers. Squeeze the brake pads against the rim, loosen the lock nut and turn the adjuster. When correct (2mm) re-tighten the nut. There is also a cable adjusting barrel on the brake lever. If you can't get it right, you may have to adjust the cable length--undo the cable anchor bolt and pull the cable with pliers. Re-tighten and test.