Choose a diet that is regarded as healthy and is meant for an active teenager.
Describe the diet and follow it for a week.
Keep a journal or a logbook. Start by writing about your feelings, expectations, fears etc before the diet.
Then write entries every day describing your feelings.
At the end of the week, write a summary of your diet, write about what was good, what was difficult, why you would or would not recommend this diet to others.
Add two or three healthy recipes, that you have tried and found delicious.
about 900 words; that is two and a half to three pages double spaced.