Riding a bike will help you lose weight but it may not come off your butt and hips right away. You can not spot reduce. This means you can not try to work a specific area and hope that the fat comes off just that area, it comes off equally on your body. In terms of working your legs out, it depends on what kind of riding you are doing. If you are placing more resistance on your ride then you will be working those muscles more then if you were simply trying to achieve weight loss.
When riding your bike the goal should be to control you heart rate. For fat loss purposes you want you want your heart rate to be at no more then 75% of its max. If you are going for endurance training then it would be 85% and above. At no more then 75% this stimulates adrenaline and noradrenaline which promote burning fat for fuel.
It is not how many calories you burn during your workout, its how long you do cardio for. If you can perform cardio for 45 mins a day it will elevate your metabolism for up to 3 hours afterwards. If you ccan do cardio for 60 min, it will elevate your metabolism for as much as 7 hours. This means you are burning more calories through out the entire day which is more important.
Good Luck