Okay let's establish this first before we get anywhere. You do not eat enough. Before you try to start getting big weight gain, you need to go get checked to see if you have the issue of hyperthyroidism. If you do, I don't know why you're eating salty foods (Unless you're using it as an excuse for high metabolism). Assuming you don't actually have an issue we'll get to weight gaining.
You need to count your calories, clear and simple. Many beginners will ALWAYS overestimate, I did this myself when I was in high school. You simply do not eat as much as you think you do. You need to find out what your total daily energy expenditure is (Use an IIFYM or TDEE calculator, simple google search) and find out what your maintenance calorie intake is. You want to eat above that, by how much depends entirely on whether you'll be working out as well, how structured and disciplined you will be with your diet. Use an app like myfitnesspal for convenience and log EVERYTHING you eat. Buy food scales and measure out portions if you have to.
At your weight and height, you need to just eat whatever you can and whatever your appetite can fit in. 115 is quite underweight but you shouldn't immediately start eating yourself sick. Measure out your portions, make a meal plan and gradually increase your calorie intake. Drink things like milk to help expand your appetite, don't drink water during meals (takes up space in the stomach) but afterwards instead. Eat calorically dense foods like peanut butter and there's a million other things you can do.
The logic behind eating more meals is you have smaller portions, but you eat more often. So rather than 3 big meals a day, you have 6-8 meals that are maybe 1/2 the size of a regular meal. Eventually your body will adapt and require food every 2-3 hours (or however you want to structure your meals). This is a common strategy for many people to use. Use something like "eatthismuch" to plan out your meals.
Eating sweets gives your body an insulin spike that helps with the carb intake and for a little bit of energy. Have some sweets after a meal, fruit is good.
Options for working out depend entirely on your goal. If you're looking to get "big" then any hypertrophy based routine will work fine as long as you keep eating and keep working out to prevent getting fat.
You need to hold yourself accountable and not bullshit yourself with excuses though. If you're gonna be serious about it, you need to commit to it.