Drink a cup of light honey water as soon as you get up in the morning, which is very nourishing and can supplement many vitamins. An egg a day is also indispensable!
Exercise, you can go jogging, run a few laps every afternoon, just run slowly, just keep running.
You can practice basketball if you like. Size is not an issue. You're just a hobby, not a competition. Coach has no reason to refuse you! Practice basketball as you like, and don't be influenced by other people's ideas!