best opening for a beginner?

the best opening for a beginner is surprisingly no specific opening. Develop your pieces towards the center abd try to achieve a pawn duo, many people say this, but it's said because it is the truth. . .check out chess network's beginner to master series where he touches these ideas: https://www.youtube.com/playlist?list=PLQsLDm9Rq9bHKEBnElquF8GuWkI1EJ8Zp

There is certainly no definite answer, but as White you should try playing 1. e4, followed by developing moves such as Nf3, Bc4, 0-0 etc. Of course, if your opponent allows you, you should seize the center with e4 and d4, for example 1. e4 ... e6 2. d4. The reason for all this is that this way the fight often starts immediately and the position is often "open", so that's why these openings are called open games. So, develop, develop, develop, castle and think of your own security before you launch an attack. 

As Black, if White plays 1. e4, you could try the simple Italian setup or the Ruy-Lopez (1. e4... e5 2. Nf3 ... Nc6 and now 3. Bc4 is the Italian game, whereas 3. Bb5 is the Ruy-Lopez.) Another option would be the sharp Sicilian defense - 1. e4 ... c5, which typically continues 2. Nf3 ... Nc6/e6/d6 and 3. d4. 

If White plays 1. d4, you can respond with 1. d5, followed by, say, 2. c4 ... e6 3. Nc3 Nf6 (Queen's gambit declined). 

All in all, you should strive for lively positions where you get an opportunity to attack, even though it may cost you many lost games. Inexperienced players should first master open games before switching to closed ones (when White plays 1. d4, roughly). Of course, as you progress, you can include all sorts of other openings to your repertoire.

See some example games and keep in mind that playing casual games is extremely important and you should play as much as possible and not learn opening theory right away. 

Sicilian is good. It helped me improve from about 1000 strength to 2000 strength and I still employ it with success.

I added it as an option because a prominent chess coach from my country said he advised all beginners to play the King's Indian and Sicilian as Black in order to sharpen their attacking ability, but IMO the KID is much more difficult for beginners to play than the Sicilian so I left it out, 

There are many levels of beginners. At your level, which is a complete novice to the game (no offense - everyone starts at the beginning), you shouldn't worry about chess theory - you just need to play a lot of games, and develop a sense of the game.

The only thing you need to worry about is not hanging your pieces. 

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 Soccerkam 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/the-perfect-opening-for-the-lazy-student
https://www.chess.com/article/view/how-to-understand-openings
https://www.newinchess.com/Shop/Images/Pdfs/9035.pdf
https://web.archive.org/web/20140627110453/http://www.chesscafe.com/text/hansen169.pdf
https://www.newinchess.com/Shop/Images/Pdfs/9029.pdf
https://www.newinchess.com/Shop/Images/Pdfs/7277.pdf

The term "chess theory" is certainly being used incorrectly here by a few people.  Learning "chess theory" means learning that the opening is a fight for space, tempo, and material: you should try to control the center, you should find move your pieces only once, if possible, as you develop. You should castle early.  You should overprotect you pieces as much as possible.  You should make threatening moves if they don't make your pieces less safe and don't cause a response that results in an improvement in your opponent's position.

That is essential knowledge for a beginner!

There is nothing inherently wrong with ideas in the Sicilian for novice players except that sometimes black ends up developing slowly as he or she decides where to best put their minor pieces.  Black needs to be comfortable playing against the anti-Sicilian lines and if white is also a novice then as with any opening be comfortable simply hanging on as white tries wacko out of book attacks.  The latter is pretty standard for novice and blitz so might as well get used to it. 

As with any wing opening, novice payers may not be familiar with the ideas so maybe there are opportunities for a few won games.  

It's all tactics for  the next 1600 ELO points anyway.  Have fun and play through some masters games while you're at it.

No - that's learning general principles. 

"Theory" (chess being of course, redundant in this sense) is the sequence of established moves int he opening - and in this day and age, it's often determined by computers. When we say an opening is theoretical, it means that these lines are often very sharp (which means that there are few acceptable moves for each side in these lines) and long (many moves of these sharp variations), which makes the game hard to play, unless you know the theory --- knowing only"develop" and "castle" and those general principles will get you into a worse position very quickly, because that's not what black does in the Sicilian Najdorf typically. 

...h5 is a common theme in ...e5 Najdorfs, which breaks a ton of principles, because black doesn't want to allow g4-g5, like in the Scheveningen Keres Attack. 

From what I've heard, you should first strive for open positions and play classically (fight to occupy the center, not control it from afar). So usually e4 as white at least. The Italian and Scotch are decent classical openings that are pretty easy to understand, the moves are very natural.

QDA? lol

It stands for the Quirky Dinosaur Attack where you move a6,c6,e6,g6 so your pawns look sharp and dangerous.

I'd say it's the Ruy lopez morphy defense exchange variation King's bishop variation  

gotta love the Queens Damnbit