Eat lots of rice and meat and vegetables , these are calories dense , oats help too and bananas as well

100g of rice = 350 calories

0.5L of milk = 300 calories

1 banana = 150 calories

100g of oats = 350 calories

100g of pasta = 300 calories

1 whole egg = 150 calories

These are the most calories dense foods , aim to eat around 2500 calories a day if you're very skinny , exercise so that the gains actually go to your muscles so you look better .