Do you know their names. It seems like you could just look them up on here and see what they play, or on lichess.
how do I prepare against someone?
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Prepare yourself, do not prepare against someone.
Good use of your time is outdoor physical exercise like walking, running, swimming, cycling, tennis. The brain is an organ and lungs, heart need to support it.
Do not focus on opponent's weaknesses, focus on your own weaknesses.
For example if you play Queen's Gambit and you know one opponent always plays Albin's Countergambit and you have no experience with it, then it might be worthwile to look it up beforehand. Do not overdo that: you might not face that player, or you might face him with black, or he might change his opening choice. I also recommend playing training games with the same time control against a strong engine. It is important that you are used to the time control: playing too fast or playing too slow may harm you. A strong engine is an ideal sparring partner, as it mercilessly lays bare your own weaknesses.
what things, in particular, make good use of my time. for more context im in a tournament hosted by a high school on lichess every thursday. only around 10 players consistently that show up so with the time in between the week there is plenty of time to figure out weeknesses for most players. The only problem is that i dont know how. If anyone has anything to help it would be greatly appreciated.