I think a guy of your height should be at least 160-180 for a healthy weight. It depends on your build and your BMI. You are still growing, so a lot of what you eat might be going to that.
I think diet is your main concern here though. Even if your thinness could be attributed to a high metabolism or thyroid, the food that you eat isn't helping at all. Soda depletes your system of vitamins. I glad that you realize that everything you listed up there is basically junk food. These things metabolize quickly and basically give you a high on no hunger and energy and do nothing for you.
Definitely look at options for a healthier diet. Whole grains will definitely help you gain weight in a healthy way. You would do well to eat lots of lean meats, but make sure you keep lots of fruits and veggies in there too because you don't want to become constipated of vitamin deficient.
Of course head to the gym as they suggested too, but you will quickly become weak and not have the energy to build mass if your doing it on a poor diet.