If I can get you to believe one thing, it's this--diets fail.
Don't seek out a diet. If you want to weigh 140lbs then you need to adopt healthy eating habits of a 140lb person that you can maintain for.....ever. It's not about crash dieting or paleo or carb cycling or whatever you find that's popular. To keep weight off you have to change permanently. Killing yourself for 6 months to reach your goal and then thinking you can stop will lead you back where you started.
You're 17. This is a marathon. Strive to lose 1-2 lbs per week. In theory it'll take you anywhere from 3 to 6 months. I'd rather it take you longer if that means you'll still be 140 when you're 27, 37, and so on.
Weight management and body composition are 90% nutrition. Exercise is a bonus in the early days. If you're on this site then you know muscle is important to most of us. A good lifting program can add muscle which will make you look better but also increase your base metabolic rate (you'll burn more calories each day). You don't have to do anything fancy; just find a program that you enjoy.
Congrats for taking charge early in your life and good luck! We're here to help.