About a year ago I weighed 104 pounds. A year later, I'm 120 pounds and my body fat percentage has stayed the same. The first thing you should do is hold off on the cardio. I completely stopped most cardio workout for a year, and am finally able to get back into it and get back into shape. Second is to start weight lifting. I would recommend finding an experienced friend. Work out at least 3 days a week, for around an hour and half of just lifting. Each day would be a different pair of muscle groups. I suggest triceps and chest one day, back and biceps another day, shoulders, legs and abs the final day. Hopefully you'll have a friend to teach you workouts, if not I suggest asking a personal trainer. Your appetite should skyrocket after this. Eat alot, but still eat healthy! Focus on lots of carbs and protein. Something that worked very well for me was adding on a peanut butter and jelly sandwich to each meal. Not only is it loaded with carbs, but its healthy as well. Also, focus on dense foods like bread. After 3 months of this you should start seeing changes. After 3 months you should also switch it up, and start going 4 days a week. Chest one day, triceps and biceps another day, back and shoulders one day, and legs and abs for the final day.
A year ago I started off at 104 lbs benching 95 lbs max. Now I'm 120 lbs and I bench 165 lbs. Muscle is the only way you'll gain weight, as you're probably like me and have a super low body fat percentage. FYI, I never took any weight gainer or protein. However, I also go to college and eat at our buffet dining hall so I can get huge meals whenever I want. Unless you are in the same position as I was, investing in protein may not be a bad idea (weight gainer is pretty damn expensive, protein isn't that bad). Just keep at it and you'll see results!
