Most brake performance issues can be resolved with just a bit of cleaning and a proper mating with the pads. You'll need a lint-free towel and some rubbing alcohol; I generally use paper towels for this.
The first step is to clean the rotor. Soak the rag in alcohol and clean both sides of the braking surface with it. You can do this with the wheel installed or removed.
Next, remove the wheel, so we can access the pads. Fold the towel over to make it thick enough so it just barely goes into the brake caliper where the rotor was, and pull it back and forth, as if you were flossing the caliper.
Reinstall the wheel and go for a ride to bed in the pads. In a safe area, get up to a good clip say 12-15 mph and apply the brake firmly, but do not lock it up. Do not stop all the way, but slow way down and get back up to speed. Repeat this process 10-20 times until your brake is working well; you will feel it get stronger as pad material is deposited on the rotor.