Okay so, the good news is that gaining weight is kind of easier than losing fat because you just need to eat more calories.
The bad news is that gaining weight means you need to eat more calories, and that means you’ll need patience and consistency, which are boring but effective.
This article explains a basic way to set up your nutrition. You’ve got to eat enough 7 days a week, and keep it going week after week. 15kg is a lot, even though you’re legitimately underweight right now. I’d set a goal of at least 6 or 7 months. Trying to force-feed things much faster is likely to end up with mostly fat gain.
How lean are you right now? Do you have any muscle definition (abs, quads, etc.)? That’s also going to influence how quickly you can/should put on size.
As far as supplements, Metabolic Drive is a great addition. It’s a high-quality protein that you can blend up with milk, fruit, peanut butter, or whatever else you feel like to make some high calorie shakes to have between meals, not instead of meals. Having Surge Workout Fuel during workouts will also let you recovery optimally and push hard during training.
For the training, believe it or not that’s sort of the lowest priority in getting the results you’re after. Any good plan will be fine. Something like this program or this 3-day routine are good choices to start with.