How to gain weight.i do eat plenty of nutritious food but still im so skinny and underwreight.

Big picture: Human body builds vary tremendously, and most of the world will envy a slim young person. "ideal weight" tables are subscience. If you can run a mile, or up four flights of stairs, or swim laps in a pool, and you feel good, perhaps this is the right body habitus for you. Don't garbage-up on empty calories. If you have unexplained weight loss, get with your physician.