How should I lose weight/stay fit this summer?

What should I do? I was very determined for losing weight after people called my fat in my first year of high school (this year). I've been called fat since elementary and I didn't really care. I'm a really lazy person. Once I find a funny show on the computer, I watch it and I don't want to stop watching it. I eat Grapefruit almost everyday and I use to do sit-ups everyday like a month ago, but I rarely do it now. I really want to lose weight, but I'm lazy. Later in the day, I always tell myself that I should run every morning when summer break starts, but after I wake up, I'm lazy and I don't want to go out sweating and just stay home watching shows. I feel like I should use my extra time watching shows and playing games since I'm kind of busy everyday too even in the summer. I have to go to a math tutor and do the homework and prepare for my piano exam coming up soon. What should I do for a person like me? How can I fix this? I sleep at around 2-3 o'clock on Fridays and Saturdays when I still have to go to school and all the time in the summer. Does that affect my laziness? My sister is even more lazy, she sleeps almost as late as me and wakes up about 2-5 hours later than me. I wake up at about 9-12 o'clock, it depends, but I don't know how it depends tho. I eat a lot, but I'm avoiding chocolate now. I only eat like a piece of Meiji's Chelsea Candy and maybe a piece of gum everyday. I love juice, but I add like about 1/3 glass of water every time I drink juice. Suggestions please :)