Learning Defense Theory

Since I'm just about getting serious with chess, I wanted to improve my openings. So, I sat down and thought for a while and came up with a plan. I did some research and quickly realized that 1. d4 is the opening for me. I then proceeded to watch a few videos online and I now understand a few move sequences and ideas behind some of the popular lines.

However, I find myself struggling with black. I know how to study specific openings, but I don't know how to approach studying defenses. For example, if I start as white I can just play 1. d4 and be happy for the rest of the game, but if I'm black and my opponent plays some line that isn't very popular I begin to struggle.

How do I go about studying the theory for black? Because there are way too many openings to learn all of them.

It's not different. The same apply it's for playing White pieces. You think 1.d4 makes you happy because you don't understand the amount of theory you should know to support it efficient. The answer is somewhere between playing a lot of long time control games and solving-studying tactics , motifs, pattern recognition and modern strategy. Before all this spent some months or years understanding pawn structures.

Don't panic! and don't ask immediate results. Learning chess is a long time process and Humans have a long life. Eventually in ten years from now you will know more.

Attack and defense are two sides of the same coin. Some of the best and most stubborn defenders in the history of the game have also been some of the most imaginative attackers. Always remember, white's goal is to widen the advantage given by having the first move and black's is to equalize that advantage. At your level, theory isn't as important as principled play. As long as you play logically in the opening and avoid creating gaping weaknesses in your position, you should come into the middlegame with a decent game. It takes practice, and lots of it. The more you play, the greater variety of positions you'll feel comfortable in. Once you have that basic grasp of the game and your games aren't decided by dropped pieces or simple overlooked mates, you'll be ready to start studying a bit of theory. Just my opinion.

For someone seeking help with choosing openings, I usually bring up Openings for Amateurs by Pete Tamburro (2014).

http://kenilworthian.blogspot.com/2014/05/review-of-pete-tamburros-openings-for.html
https://chessbookreviews.wordpress.com/tag/openings-for-amateurs/
I believe that it is possible to see a fair portion of the beginning of Tamburro's book by going to the Mongoose Press site.
https://www.mongoosepress.com/excerpts/OpeningsForAmateurs%20sample.pdf
Perhaps RohilNarula would also want to look at Discovering Chess Openings by GM John Emms (2006).
https://web.archive.org/web/20140627114655/http://www.chesscafe.com/text/hansen91.pdf
"Each player should choose an opening that attracts him. Some players are looking for a gambit as White, others for Black gambits. Many players that are starting out (or have bad memories) want to avoid mainstream systems, others want dynamic openings, and others want calm positional pathways. It’s all about personal taste and personal need.
For example, if you feel you’re poor at tactics you can choose a quiet positional opening (trying to hide from your weakness and just play chess), or seek more dynamic openings that engender lots of tactics and sacrifices (this might lead to more losses but, over time, will improve your tactical skills and make you stronger)." - IM Jeremy Silman (January 28, 2016)
https://www.chess.com/article/view/picking-the-correct-opening-repertoire
http://chess-teacher.com/best-chess-openings/
https://www.chess.com/blog/TigerLilov/build-your-opening-repertoire
https://www.chess.com/blog/CraiggoryC/how-to-build-an-opening-repertoire
https://www.chess.com/article/view/learning-an-opening-to-memorize-or-understand
https://www.chess.com/article/view/3-ways-to-learn-new-openings
https://www.chess.com/article/view/how-to-understand-openings

https://web.archive.org/web/20140627132508/http://www.chesscafe.com/text/hansen173.pdf

https://www.newinchess.com/Shop/Images/Pdfs/9029.pdf

"Since I'm just about getting serious with chess, I wanted to improve my openings."

This statement reflects one of the many ironies of improving beginners in chess. 

I only wish the title of this thread were literal - studying basic defense (defensive tactics) is probably the #1 thing a beginner can do to get better, apart from learning basic tactics.