I am planning to join a gym.what exercise should i do to lose weight? i'm overweight by around 15 kg.at what speed should i run on treadmill?how long?

Gym: A great idea. Almost all gyms have plenty of exercise trainers who will work you through everything you want to do. I am already old (70 and slowing down) but have had wonderful experiences at my club. ENJOY!

Most important thing: is consistency. Just pick a machine that speaks to you, and start using it, without high-pressure expectations. Low impact machines such as elliptical is preferable to treadmill for the reduced wear on your joints. Start with 30 minutes minimum, and increase to 60. Start with zero resistance and work up slowly to your comfort level. You want the workout to feel good, not like a chore.

Remember: Exercise is great and has so many benefits. But for weight loss, the key will be for you to be mindful about reducing your caloric intake. Exercise can help augment caloric restriction. As others have said with excise find something that'll do consistently. In terms of intensity, cardiovascular wise try to exercise at the level that's difficult to hold a normal conversation.