Yes cycling will help you too lose weight and tone your legs and butt. You can not spot reduce any part of your body. The weight comes off all over at the same time. Most people will notice a weight lose in the face because it is exposed.
You want to pedal at a high cadence 70-90 rpm's with a lite pressure on the pedals. That will use your slow twitch muscles that use fat as a fuel. If you mash on the pedals you will use glycogen as a fuel and will cause you muscles to burn and not last as long.
You need to reduce your calorie intake as well as exercise. You want to start out slow and just work on distance. You will get faster as time goes by and you get stronger. The better the shape you are in the more calories you can burn.
http://www.caree.org/calories.htm