This is going to be long but, I know your situation and I’m going to try to help you in the best ability that I can. First and foremost, check your BMI and your BMR. What a BMI is, is a body mass index. It tells you based on your height, weight, and age if you’re underweight/normal weight/overweight. Next is your BMR which is, basal metabolic rate. It tells you how much calories you burn every single day based on height, weight, age and if you’re a male or female. Lastly is this website, it tells you how many calories to eat everyday. 

http://www.freedieting.com/tools/weight_gain_calculator.htm

Now, when I started gaining weight I started at 109 and now I’m around 117 or 118 after a month and a half with doing this and that’s great for me because gaining weight for me is extremely hard (I burn like 2000 calories everyday and I have a very small stomach, sucks lol). So with that last website I gave you, it told me I had to eat 2800 (around) calories a day to gain weight. So I literally put in 2800 in a calculator on my phone every morning and I count to see how much I ate that day. Like for example today, after dinner I will have successful ate 2800 calories all together day aww yeeaah lol. Eat a lot of protein and work out! Yes working out does help and it makes you look bigger (with the muscles). This isn’t going to be easy, it’s going to be tough but it’ll be well worth it in the end when you see how far you’ve come. Good luck, I know you can do it. :) x