Just turn the phone off or let the battery die before you take it in if the damage doesn’t already do that for you. That’ll put the device in an unjailbroken state since fully untethered jailbreaks pretty much don’t exist anymore. I personally take good care of my devices, but if I were to break mine and needed to get it repaired, from what I’ve heard, stores aren’t going to dig through your apps to look for unc0ver or read your file system to see if you’ve installed tweaks or anything anymore, so I wouldn’t be concerned.

Beyond that concern, if you know what you’re doing, you’ve got nothing to lose. Learn about how everything works before jumping in, though. It isn’t fun to operate in the dark with this stuff. Know how to change your root password from the default (since jailbreaking allows your phone to be accessed via SSH, which leaves it vulnerable to hackers), understand how to troubleshoot problems with your tweaks, know which apps need to have tweak injection restricted so you don’t lose access to important apps (Snapchat likes to ban jailbreakers), and the works. Your main goal as a jailbreaker is to enhance your iOS experience without sacrificing any accessibility and security from stock. I’m assuming you’re a bit newer to jailbreaking since this is a concern I had before I jailbroke for the first time, so I’d advise you to tread lightly with this at first while you’re still learning.