Chess Preparation
What is "Chess Preparation"? In almost 2 years of play, no one has come to me with a single suggestion on that topic and in fact tournament players go out of their way to hide their preparations for a tournament!
On Chess.com though, preparation is often somewhat different than tournament or match play. For starters, most games on Chess.com are with unknown people, although you can control for opponent rating level when offering a game. Preparing for that sort of a game is a lot like a box of chocolates.

So you stick to the basics.
I've found having a small number of game systems that you're very familiar with is helpful for this sort of a game. A tournament or match, against an known opponent with a recorded game history involves a bit more specific study in advance.
You'll need more than one game system though, particularly depending if you get White or Black pieces. If you get the White pieces you control the start and 1. e4 or 1. d4 are most common and easy to stick to. I tend to start White with 1. d4 because that immediately stops Black from the Sicilian or the Caro-Kann, and maybe Black now has to do some thinking.
If you get Black in your game, you need more options because you don't control the start. This is probably why I don't like playing Black so much. I've had to learn much more, to be effective in those games.
My next point is also a Chess Gem. You should be able to win with a system, as well as defeat it, and that goes beyond memorizing moves. Your opponent may walk off your memorized game movement line, and now you need to start thinking. So start work on understanding your system, not memorizing piece movements. That means you don't always need a lot of theory, just a good understanding of pieces interacting on the board.
You do want to stay up to date though. The London System is a good example. Recently, in Chess terms, that system has shifted towards something called The Jobava. If you're a London player, you may like to look that one up.
Next, and this can be hard. If it's not working? File it. Put it away for another day. I spent a bunch of my recent beach vacation studying to actually play the Sicilian and not just defend against it, and it's hard! I'll come back to that one later.
So in spontaneous games against unknown players, do what works for you.
This is quite different from what I've seen and done in tournament and match play, where I know who my opponent is going to be. With that type of play, I regularly go back to my opponents recent games to see what they often do, do well, or don't do so well against.
I just did that with our current Team Canada vs Team Spain Match

I noted that my opponent was often simply not seeing diagonal moves or attacks, so I brought out ideas to take advantage of that and have already won the first game, with the London System, and the Checkmate was delivered by a Bishop.
https://www.chess.com/game/daily/596077495
The second game, with me as Black is almost done and I did much the same. I was helped by my opponent Blundering his Queen, but diagonals have been a feature in that game too, and I will do a more in-depth post about it when that game is done.
Please tell us how you prepare in the comments. New players need to learn too!