The most often recommended way to build muscle, regardless of age, is through plain exercise, specifically Weightlifting. Combining weightlifting with proper nutrition, hydration, and rest can produce the best results. Some supplements exist but are not proven, and can produce dangerous results.
There are a lot of good teen books out there, but some really good ones are Alyzon Whitestarr, The Hermit Thrush Sings, and The Clique.
Some good teen costumes ideas is a bunny,cheetah,zebra,and nerd
Justin Bieber
A teen could benefit from exercise by strengthening muscle tone.
Teen relationships are very important and even if you are not a teen if you are 10-19, they are very important to help you build social skills. Also you will understand how to pick out good friends and not run with the wrong crowd.
you can fine some good teen programs and their schedule in the following link http://www.toledolibrary.org/events/teenprog.asp#tp
www.rnr-advice.weebly.com
Delias and Alloy are goo.
what is a good idea for a bedroom design
About 17 ounces a day
YMCA are really good starting point for Teens. If the teen does like sports, they can channelize their energy in a sport of their liking. It also helps them to focus and build up self esteem as well.
I think Imvu is fairly good and maybe weeble.