The best way for you to get your body used to a physical activity is to do some kind of martial arts. If you can find one that will make you sweat and constantly push you, like Muay Thai or Judo (from my experience), it will make it much easier on you to actually lose the weight while keeping your muscle tone.
Dieting alone won't do the trick, but you need some activity that will shape your body. And there is nothing better to shape every part of your body than martial arts. An added bonus is that you will train with other people that will eventually become your friends and that will push you on during your workout sessions.
After a few months of training like that, your body will adjust to the workout level, and then you can go to the gym and go through the regular routine. But I think starting off, without having any exercise, and going running or biking is not the way to do it. You won't get anywhere close to the calorie burning level in the beginning, and most likely will give it up after a while since it's very discouraging seeing people outrun you without breaking a sweat.