I listened to some of the best advice for putting on weight and it was go to the store buy peanut butter jelly and a loaf of bread. And what pbj all day long lol and lift weights as often as u can
Carbs+protein+lift heavy = size gains
Gaining weight healthily would require you to up the number of calories you're eating (mostly deriving from good fats and protein)and weight training.
You can either gain muscle weight (which is what I just gave u advice to) or fat weight which in that case you will not look anything but fat.
Every body is different and if you're naturally thin don't try to gain fat it won't look normal and you won't feel like normal(you won't be healthy or feel healthy or happy or energetic) . If you're trying to tone up and therefore weighing more than go for it and good luckk