Need help understanding how to create a repertoire and my own database

I’m a 2100 ECF rated player (equal to 2100 FIDE) with 3 years of chess experience and I have never bothered in finding out how to create a repertoire. I think now is the right time to start, but I’m not fully sure of how to create a repertoire online. Let’s say I want to put the Sicilian Taimanov in my repertoire. And a few moves after reaching the Taimanov position, I have a choice of two or three moves showing up in the database that 2500 ELO players play. Do I just choose the most played move by GM’s and put in in my repertoire or should I make variations for the top 3 moves played by GM’s? 

Also I’ve seen a lot of people online saying you should create your own database and add your games to it. How does that work and what’s the reason for adding games I’ve already played to it?

What do GM’s use for saving their repertoire?

The openings you decide to play should be based on the types of middle game positions you're comfortable playing.  Hopefully you're dong that.  As far as a database?  I use chessbase 15

You should definitely use chessbase if you are serious about chess. That's what all GMs use.

When building your repertoire, it's up to you how many different lines you want to have in your repertoire. If you play serious tournaments, it's better to have more than one, so you are less predictable.

And you don't have to follow the most played GM (or top engine) recommendation everytime. Sometimes these moves lead to positions that are difficult to play. Make sure you choose moves that lead to positions you are comfortable with.

In my opinion it's also important to consider some other factors when building a repertoire. Do you want a sharp or quiet game? For example, if your opponent plays something sharp, you can choose a line that's more solid. If he plays something quiet, you can try to find a sharper continuation. This way you increase the chance to reach a position your opponent is not super comfortable with.

Or sometimes move A is slightly better but move B makes it easier for your opponent to go wrong. It's your decision if you want to play the "correct" move or the move that gives you better practical chances.

There are different approaches and therefore you shouldn't follow anyone's recommendations blindly but use your brain and choose whatever you think works best for you in practical games.

You got to 2100 ECF without a repertoire, why do you think you need a repertoire now? What did you play as black against 1 e4 and against 1 d4 until now and how did you open as white until now? The best way is to study your own games and also study annotated grandmaster games that opened the same way as you play.

I was playing different openings as BLACK pieces without knowing much about them and I think I could have been way higher if I studied properly. I used to lose many games against similar rated opponents because I knew less about the openings I played and I couldn’t get into a comfortable position. I never created a repertoire because I was too lazy to do it. So now I am doing that 

Thanks for the advice, I’ll watch a YouTube video on using chessbase for repertoire. I’ve been looking at different GM’s games and the middle game position they get after the opening and I like the types of positions Gukesh D gets.

#5
"I was playing different openings as BLACK pieces"
++ Do not play different openings, stick to one to accumulate experience
"without knowing much about them"
++ That is all right, just think of your own and accumulate experience
"I think I could have been way higher if I studied properly."
++ No, that is not right. Study of openings is ballast as Nimzovich called it. What you study does not happen and when it finally happens you will have forgotten.
"I used to lose many games against similar rated opponents because I knew less about the openings"
++ That is probably false. Look at those games again. You hardly ever lose because of the opening. You generally lose because of middle game tactics. Proof: play the good side of a bad opening against an engine
"I never created a repertoire because I was too lazy to do it."
++ That is not lazy, that is smart. There are much more rewarding activities than opening study. Endgame study for example.

Do you think the modern chess openings 15th edition is good to use?

What you’re saying is I shouldn’t study openings at all. You haven’t even seen any of my games so how can you say that? It’s true that sometimes I would get into positions in the opening and play an inaccuracy because I didn’t understand the opening, which led me into an inferior position. I can even show you some of my games if you like. You don’t even know what you’re talking about, if someone has a worse position after the opening or in the opening, that gives a big disadvantage!

Just think about what you’re saying. It’s like a serve in tennis

For example, in the Ruy Lopez. At a certain point in the opening there is only one or two good moves you can play, but I didn’t know what to play and ended up playing a huge inaccuracy or a move which gave my opponent a huge advantage. Like me playing a move which led my position to be passive the whole game.

Yes, I had the same question before. I still haven't created an opening repertoire (it is a daunting task and I've been busy as well), but this thread may be of use for you to glance through:

https://www.chess.com/forum/view/chess-openings/saving-opening-repertoire 

#11

"What you’re saying is I shouldn’t study openings at all." ++ Indeed, you are better off if you do not study openings. Just play the same opening all the time, learn from your mistakes, if you want look it up after you have played.

"You haven’t even seen any of my games so how can you say that?"
++ It is true for everybody. If you want proof, then play the good side of a bad opening against an engine. The engine wins despite the bad opening imposed on it.

"It’s true that sometimes I would get into positions in the opening and play an inaccuracy because I didn’t understand the opening, which led me into an inferior position."
++ You lose because you make a mistake, not because you did not know.

"I can even show you some of my games if you like." ++ If you show a game that you lost then I could point out what you had to do so as not to lose it.

"if someone has a worse position after the opening or in the opening, that gives a big disadvantage" ++ If you play healthy, solid moves then you do not get into a worse position

"It’s like a serve in tennis" ++ Here is my favorite example. Marshall had secretly studied his novelty 8...d5 for years; for Capablanca it was a total surprise. Guess who won...

https://www.chessgames.com/perl/chessgame?gid=1095025 

?? What? That doesn't work. If your opponent plays an opening line that you never studied, you're almost certainly going to lose.

"?? What? That doesn't work. If your opponent plays an opening line that you never studied, you're almost certainly going to lose."

More HORRIBLE advice.  Openings have no impact on your games.

I mean, if my opponent plays a gambit,  that requires me to play an exact move, it could lead me to a worse position if I don't know what I'm playing. 

Ok so now we have moved from on openings to gambits.  What gambit are you referring to?

I never used it so I can't say much about it. But see it's from 2008 and since then the theory of many variations has changed.

For example if you study the Taimanov, one of the critical lines nowadays is Nc6 Nc3 Qc7 Be3 a6 and then Qf3. This Qf3 move didn't exist in 2008 so it's most likely not covered in the book. 

I think in general the book is okay but you should also use other sources like databases in addition

Danish gambit I believe? 

First of all, send your opponent a thank you card for giving you 2 pawns.  Then?  Just play normal developing moves.