Building muscle, and loosing weight?

[i Can <3 WE can]

I plan on loosing weight over the summer and fall. I was wondering what kind of excercises and foods can I eat to loose weight AND build a litlle bit of muscle. I am in an office all day (from 7-5) and so I was wondering what excercises can I do inside but not videos. I do not work in the office but I have to go there everyday because of my parents. I am kind of young, (13 years of age,) but was still hoping you could help. I want to do something healthy and natural that will help me later in life. It has to be affordable, though, since being I am so young. I also do NOT want it to be something like surgery, pills, or anything dangerous, etc. I want to loose at least twenty five pounds. Please help me, but have considerate and responsible answers. Thank you.
I am doing this so that next year I can learn about some sports such as: softball, soccer, basketball, cheerleading, gymnastics, dance, and much more. Seeing as I need endurance and a healthy stamina, I am asking this question.