I recommend eating healthy and get rid of junk food. But very hard to avoid junk in Malaysia (cendol, nasi lemak, etc).

I highly recommend Scooby's workshop as your source of info. A lot of things in youtube but can't trust all of them:
http://scoobysworkshop.com/

He has a few kinds of calculators for you to find out how much of food intake is needed depending on your goals. Of course, wanting to gain weight means you have to eat a lot more than your current diet!

The calculator will tell you how much carbs/protein you need to gain weight based on your current weight.

Since you have to eat more, don't just rely on 3 meals a day, which means you'll have eat a whole lot in one sitting. My advice is to have at least 5 meals (or 3 main meals and 2 mini meals) in order to hit your target carbs/protein intake. Ideally it would be 5-6 main meals but I assume we haven't reached that extreme yet

For me, I know my protein intake cannot be achieved from food alone so I have to take whey protein powder as supplement.

And don't forget to use those muscles/workout/exercise/play sports. Otherwise, those extra food you consume will just turn into fat.

If you lift weight, also important to learn about when is the best time to take protein, etc. It's a long journey to understand all these things but definitely worth it!

This is all from experience. I started gaining weight February this year. Managed to gain 10 kg and maintaining so far

You have to examine the nutrition info label and decide if you need it or not. Most of the time, you can get the nutrients from cheaper source like rice. The weight gain powder formula is targeted for those who wants the simplest way to gain weight and money is no issue. Usually it is expensive.

If you need more carbs, eat more oats/rice and if you need more protein, eat more chicken/beef/meat. More economical this way, instead of relying solely on the weight gainer.

Just ignore what other people think. In the end, it's your body, not theirs.

I have a skinny guy in our gym. Nobody laughs at him, you never know he might become bigger than all of us in the future.

Keep your spirits high.