First, inspect your brake pads for wear. Most have a wear line to indicate how far you can push it before replacing them. If not, measure your pads against a stack of three quarters. If the pads are smaller, it's time for replacements.
Give each brake lever a squeeze. If it feels soft and mushy, unscrew the barrel adjuster on top of the brake caliper to increase the cable tension. If too tight, screw it in. And if the barrel adjuster won't screw or unscrew enough to give you the tension you prefer, you can tighten or loosen the cable itself by loosening the anchor bolt on the side of the caliper and pulling or releasing more cable.
Once your brakes feel firm and powerful, drip a few drops of chain lube into the ends of your brake cable housing and grab the levers a few more times to work it through. Nothing impairs braking and shifting performance more than dry or dragging cables.