Cheap food to eat to gain weight but still be healthy in Japan

Im 17 and Im pretty unedrweight and I want to be healthier but I dont know what to get in Japanese conveniece stores or supermarkets. Anything cheap that you recommend?

I was recommended the Kireto Lemon Mukumi (キレートレモン) to be healthier by my friends but I also want to know meal wise/ drink/protein.