I am a thin person. . no matter how many nutritious foods i eat, i do not gain weight?

See an internal medicine physician for diagnostic tests, including TSH levels and other thyroid function testing.

If you are otherwise healthy, just thin, let it be. With age you will gain weight and look back fondly at this time when you could eat and not get fat. For healthy diet: http://www.choosemyplate.gov/., exercise 30 minutes/day, drink enough water daily so your urine is mostly colorless, have safe sex, no tobacco alcohol weed or street drugs.