Where do you start with 'opening theory'?

What are the basics one has to learn?

Start with 1.e4...e5 

Italian, Spanish, Russian, Two Knights  (Learn the opening 5-10 moves deep)

Play a few games to get a feel for the opening.

and use learn openings here to find were went wrong

Perhaps it would be a good idea to start with Discovering Chess Openings, a book about opening principles.
https://web.archive.org/web/20140627114655/http://www.chesscafe.com/text/hansen91.pdf

Here are some books that set out to help the reader to choose specific openings:

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/

https://www.chess.com/blog/ForwardChess/book-of-the-week-openings-for-amateurs
https://www.mongoosepress.com/catalog/excerpts/openings_amateurs.pdf

First Steps: 1 e4 e5
https://www.newinchess.com/media/wysiwyg/product_pdf/7790.pdf

https://chesscafe.com/book-reviews/first-steps-1-e4-e5-by-john-emms/

First Steps: Queen's Gambit

https://www.newinchess.com/media/wysiwyg/product_pdf/7652.pdf

My First Chess Opening Repertoire for White
https://www.newinchess.com/media/wysiwyg/product_pdf/9033.pdf
https://chessbookreviews.wordpress.com/tag/vincent-moret/
Opening Repertoire 1 e4
https://www.newinchess.com/media/wysiwyg/product_pdf/7819.pdf

Yasser Seirawan's Winning Chess Openings
https://web.archive.org/web/20140627132508/http://www.chesscafe.com/text/hansen173.pdf

Be sure to try to use the openings in games in between sessions of learning. 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. After a game, it makes sense to try to look up the moves in a book and see if it has some indication of how one might have played better in the opening. Many opening books are part explanation and part reference material. The reference material is included in the text with the idea that one mostly skips it on a first reading, and looks at an individual item when it applies to a game that one has just played. Resist the temptation to try to turn a book into a mass memorization project. There are many important subjects that one should not neglect because of too much time on opening study.
https://www.chess.com/article/view/learning-an-opening-to-memorize-or-understand
"... Overall, I would advise most players to stick to a fairly limited range of openings, and not to worry about learning too much by heart. ..." - FM Steve Giddins (2008)
"... I feel that the main reasons to buy an opening book are to give a good overview of the opening, and to explain general plans and ideas. ..." - GM John Nunn (2006)
"... If the book contains illustrative games, it is worth playing these over first ..." - GM John Nunn (2006)
"... the average player only needs to know a limited amount about the openings he plays. Providing he understands the main aims of the opening, a few typical plans and a handful of basic variations, that is enough. ..." - FM Steve Giddins (2008)
"... Everyman Chess has started a new series aimed at those who want to understand the basics of an opening, i.e., the not-yet-so-strong players. ... I imagine [there] will be a long series based on the premise of bringing the basic ideas of an opening to the reader through plenty of introductory text, game annotations, hints, plans and much more. ..." - FM Carsten Hansen (2002)
https://web.archive.org/web/20140627055734/http://www.chesscafe.com/text/hansen38.pdf
"The way I suggest you study this book is to play through the main games once, relatively quickly, and then start playing the variation in actual games. Playing an opening in real games is of vital importance - without this kind of live practice it is impossible to get a 'feel' for the kind of game it leads to. There is time enough later for involvement with the details, after playing your games it is good to look up the line." - GM Nigel Davies (2005)
"... Review each of your games, identifying opening (and other) mistakes with the goal of not repeatedly making the same mistake. ... It is especially critical not to continually fall into opening traps – or even lines that result in difficult positions ..." - NM Dan Heisman (2007)
https://web.archive.org/web/20140627062646/http://www.chesscafe.com/text/heisman81.pdf

Chess Openings Resources for Beginners and Beyond...

https://www.chess.com/blog/RussBell/openings-resources-for-beginners-and-beyond

https://www.chess.com/blog/RussBell

Great answer thanks 

Like many other points of study in chess, I've learned them as I went. True, some endgames I studied before I "needed" to know them - but most things we've learned from seeing a need. For most players (I've observed), how do they learn what a fork is? By learning chess and figuring out that your opponent always catches you with this "trick" (usually a Knight fork and of that, it is usually forking the uncastled King and Rook xD). You then learn this is a "fork" and realize that it isn't limited to one type of piece.

In the same way, I would imagine that most chess things studied are looked at because:

1) They fell prey to it (like the above fork example) and wanted to learn this "trick" for themselves

2) They anticipate they will need to know this later on (learning more rare endgames, or opening theory could fall into this category)

or 3) Learning something impractical, but the themes at play might be applicable to another situation that is more practical (chess puzzles beginning from illegal positions come to mind as the solution motif might be applied to a real game)

With this all said, I learned most of my opening theory because I look up opening lines after I play them. Whenever I analyze a game, which opening was this? My opening knowledge slowly grows over time from necessity and repetition (usually adding one more move at a time as they come up). As many have mentioned, it is best to study mainlines and common sidelines. Making "normal" developing moves and such will likely follow opening theory anyway - it is then just a matter of remembering how the lines branch from there: which moves are sound and which ones to avoid for what reason (so if the opponent deviates, you know if you should continue with slow positional pressure or to instantly play a known refutation).

In summary, I learn opening theory mainly by my own analysis. I play a game and check what opening it is: I can then research that opening later, so I know how to handle the same position next time I get it in a game. It is a slow process of playing correctly (or often time incorrectly) and then seeing where I may be able to improve - through this long method, your opening knowledge will slowly grow one move at a time - but it is thorough and you have a better chance of remembering it.