You might tell us what you're eating now, and what you're family members look like. People are different. It is healthier to be lighter and it's healthier to eat less, as long as what you eat supplies the proper nutrition, but if you're tired all the time and you sleep well and exercise that indicates something might be missing. Have you had a checkup with your doc? The healthiest way to gain weight, assuming there's no medical reason for not gaining weight, is to eat more and do resistance training of some kind, which will build muscle and fill in your frame in a lean and healthy way. Usually to do this people will advise eating additional healthy protein foods. If you just want to gain weight and don't care if it's healthy weight, just eat more simple carbs, but I don't recommend that.
I think that first thing to know is your body type, i suppose from what you mentionned it's ectomorph that means you have fast metabolism and whatever you eat it doesn''t turn to be fat or muscle, search in google about this type there are a lot of suggestions and tips for you
I know i didn't answer your question but i hope you'll get what you want .
Good luck !