Eat A LOT. If you think you're eating enough you're not. In order to gain weight you need to be in a caloric surplus, which basically means eating more calories than you burn. I used to be really skinny as well and I've had to force food down in order to gain weight. An easy way to know if you're eating enough is to track your calories. MyFitnessPal is a good app to track the foods you're eating. You put in your info like your weight and height and it tells you how many calories you need to eat in order to gain weight.

Also, make sure you're working out. Don't just eat like crazy and sleep. A lot of people will tell you to eat specific foods and to make sure you're not eating junk food and stuff, but honestly, eat anything that you have in front of you. Obviously every meal shouldn't be greasy burgers and fries, but don't be afraid to eat those kinds of foods. It makes gaining weight a whole lot easier. Your main focus right now should be to bulk up and put on size.

Some foods to eat to gain weight:

Peanut butter
Pasta
Almonds
Potatoes
Eggs
Chicken
Beef
Milk
Icecream
Yogurts
Oatmeal

If you have questions feel free to PM me.

A good tip is don't drink a lot of liquid and if you do make sure its something full of fat like a milkshake or soda do not drink water.

Also just eat a lot of fast food shouldn't be too hard.

You should eat anything more healthy that's rich in protein. You will of course need to make sure you eat an amount of fatty foods as they are needed in lesser amounts. Going to the gym and using supplements such as Whey Protein May help you build muscle over a period of time. Just have breaks and give your self time to recover if you do. It's no good eating plenty of junk food as that's certainly bad for you and will likely just make you become overweight. Remember you want to be in healthy weight range.

Buy Apetamin Syrup and ON Serious mass weight gainer
Drink the syrup like 1-3 times a day, it might make you sleepy so i would drink when your done work and everything you gotta do for the day, drink the mass gainer everyday aswell, i would drink at night.
Workout and eat! I promise youll see results within a week!

Eat Burgers and fries for a month.

Definitely track your calories, it's such a huge help! I went from 7 stone to 11.5 stone in just under a year. If you can't eat big meals, eat little but often and also blend some food as shakes if you can't seem to squeeze them extra calories in. Make sure you're on a calorie surplus, for me it was like 600 calories more than the recommended amount.

I'd say eat more than what you usually do, eat fatty foods at night such Pringles, Takeaways and other fatty foods. Eat healthy of course but at the same time you want fatty foods bro.

I have the same issues, everyone says "eat more" he problem is I already eat loads! Personally, I managed to gain 2 stone in the last year is time. Aging is the best way for us lankys to gain weight.

I'm the same and I eat a lot of chicken, drink milk, eat icecream, even If I eat loads of McDonalds I don't gain any weight at all

I'm 20 years old, 6'2 and literally slim af I never gain weight but I don't really mind