1. Eating

Eating is the most important step when trying to gain weight. Plan out your meals, try to eat foods that will help gain lean muscle mass and stay away from trans fats. I would say eat about 5-6 meals daily. Breakfast, lunch, dinner, but most importantly try to consume a meal every 3 hours. Stick to high proteins and try to eat more calories than your actually burning.
Here is a list of foods that are good when gaining weight.

Steak, chicken, peanut butter, fish, low fat milk, cheese, and egg whites.

2. Weight lifting

W know eating is the most important step in gaining weight but weight lifting is also very important. I would say you should probably lift weights at least 3 days a week. You must lift hard on those 3 days and take the rest of the week for rest and recovery. We grow while we rest so it is very important that you rest, also too much workouts will burn all your calories and the goal is to consume more then we burn. Here is a list of some big exercises I would recommend.
Squats, dead lifts, bench press, chin ups, etc.

As you increase strength, it will ultimately help with your weight gains.

3. Supplements

No, we don't need supplements but I highly recommend taking at least one of the two mentioned below. Taking both is absolutely fine and will show increases a lot faster.

1. Weight gain shake

I recommend "Cytogainer size and strength maximizer"

2. Nitric Oxide

I recommend "Black powder nitric oxide"

The weight gain shake will help with your calorie intake. Sometimes its hard to consume more calories then you are burning.

The nitric oxide will assist in storing the nutrients that you are consuming. This is a major boost when you are eating good.