What should I do to improve my game?

1.e4 e5 as Black. Well, I am totally an amateur so I would just do some experience to figure out the best routine for me T.T

Sounds like some sort of attempted mass memorization project. I do not remember seeing much in the way of claims of success with that kind of approach. Most of the time, one faces a position with no knowledge of a specific move indicated in a book. One has to accept that as part of chess, and think of opening knowledge as a sometimes helpful aid.

"... there will come a time, whether on move two or move twenty, when your knowledge of theory runs out and you have to decide what to do on your own. ... sometimes you will leave theory first, sometimes your opponent. ... It happens in every well-contested GM game at some point, usually a very significant point. ..." - IM John Cox (2006)

You might want to think about one specific choice after 1 e4 e5 2 Nf3. If you choose 2...Nc6, you can ignore 2...Nf6, and you might next think about your personal choice of a reaction to 1 e4 e5 2 Nf3 Nc6 3 Bc4. These sorts of issues are discussed in First Steps 1 e4 e5 (a book with explanations of a bunch of games).

https://www.newinchess.com/media/wysiwyg/product_pdf/7790.pdf

 You are right! I will take into consideration your opinions to improve my training method. Chess is a lifetime study so if I can figure out an effective way of training, I will have no worry in the future.

I suspect that "worry in the future" is a part of continuing to be a chess player.

Is 1.e4 d6 a sound defense against 1.e5? Black has two possible systems: Pirc and Philidor defense, which are covered in details in some good opening books.

1.e5 is not a possible move. You must have meant 1.e4.

Aye Sir! My bad, typo in the phone

If 1 e4 d6 fails to be sound, I would be the last person to know, but, since there are books about it, it does not seem likely that 1 e4 d6 is unsound. On the other hand, it might be difficult to find relevant introductory-level books about it.

Yay! All I care about is a universal system that is not heavily analyzed by White. I don't want to deal with Sicilian or Scandinavian.

If you really want to pursue 1 e4 d6, I suppose you could look at The Pirc: Move by Move
https://www.newinchess.com/media/wysiwyg/product_pdf/7604.pdf
and/or First Steps: The Modern.
https://www.newinchess.com/media/wysiwyg/product_pdf/7700.pdf

I have no data, but my guess would be that the Pirc does not differ greatly from the Scandinavian as far as being a method to avoid analysis.

At my level, I just want to have a playable system to avoid going deeper into opening study. So, 1.e4 d6 is the most reasonable answer to me as Black against 1.e5

I don't care if it is beaten easily at 1800+ or at 2000+ because I am just an amateur of 1100+ ELO who is struggling to improve in chess T.T

If youre intersted in playing 1...d6 against 1.e4, check out the Modern Defense, and the games of Duncan Suttles.  

I'd say it's the opposite.

The pirc gives white many options, choices that lead to radically different middlegame ideas, and some of them with big attacking chances. So black has to learn not only a wide range of theory, but deep theory as well. while white has to only learn which of them is his favorite.

But like @Anhvant said, at his level it's not such a big deal... but down the road, the pirc  and Scandinavian are on opposite ends of the theory spectrum.

It is understandable to want to have a playable system to avoid going deeper into opening study. I think that it has been plausibly argued that 1 e4 e5 is better for one's long term chess growth prospects, but I have difficulty believing that it would necessarily be a major disaster if one tried 1 e4 d6 for awhile. Around 1100, I don't know that one would be likely to encounter the Austrian or 150 Attacks all that often, but, if one stays with 1 e4 d6, I suppose that one is likely to face them sooner or later. I imagine that there is at least an introduction to these possibilities in the First Steps book.

IIRC Yasser Seirawan said as a beginner he played the pirc and KID against everything... so it can't be all bad  He became a strong grandmaster.

But yeah, the usual advise is 1.e4 e5.

I guess I was thinking in terms of the Pirc not being an improvement on the Scandinavian for the purpose of avoiding theory. I guess I should have given thought to the idea that it could be a lot worse. Still, is it likely to be a lot worse for a ~1100 player?

Yeah, probably the same at 1100