HELP! i'm having trouble with my weight... (16 years old, 85kg!)?

I am a 16yr old girl who weighs nearly 85 kgs. I have been trying to lose it for years, but i lack the motivation needed. I want to change the way that i eat, and what i do in life. I usually sit at the computer, or ride my bike, and once a week i have dancing lessons. but in the past 3 months, i have stopped riding, and i am having great difficulty dancing, as my knees ache, and my ankles can't support my weight anymore. So please, if you could help me, that would be brilliant. I am looking for exercise routines, and healthy meal recipes for myself. I need to change. i am sick of the way that i am. Thanks.