Christian, I know it's been some months, but how did you feel you did on the tournament? There were a lot of ideas here, were you able to use any of them with the limited time you had to prepare?
Entering my first tournament (OTB) Need some opening tips
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@ChristianLoew Right so, for every opening, especially popular ones, both sides have theory for different plans they might enact straight out of the opening. For example, the King's Indian Defense is an opening named after black because it is sort of "initiated" by black by the moves Nf6 and g6 in reply to d4. However, in the King's Indian Defense, the theory behind it is that black will develop his pieces fast and strike back in the center later but there is different theory for white too and those also have names for example King's Indian Defense: Four Pawns Attack (variation) is the name of an opening that tells you what black's plan is and what white's plan is.
So basically if black plays a defense or white plays an attack, the opposite side has some preset ideas or plans to take advantage of what your opening lacks in.
So for the King's Indian Defense you are giving up the opportunity to establish a pawn presence in the center and therefore giving up space, so many of white's variations take advantage of the fact that you are not grabbing space by grabbing a lot of space in the center.
These variations or preset ideas and plans have been played many times and studied for many hours. I don't want you to just memorize what you should play as white against the Sicilian Defense though. What you need to do is understand what your plan should be or what you are playing for, or at least what the weaknesses of the Sicilian Defense are so you can play to attack those weaknesses.
To recap, even though openings that say defense are a black opening, and openings that say attack are a white opening, both sides have ideas of what to play in each opening and you should familiarize yourself with at the very least the Sicilian defense from white's perspective because again, a lot of people play the Sicilian in response to 1.e4