How do you guys choose what guild to join when playing, and how does guild membership affect your own gameplay?
A guild will almost certainly mean nothing to you until you hit max level (currently level 80). Nobody pays any attention to levelling or levelling characters.
IF you are raiding Mythic, a guild is essential.
If you are doing Heroic Progression, a guild is eessential for most people.
Both of these cases are like trying for prizes in football - you can’t do it without being part of a regular team.
In these competitive cases, you need to choose to join a team with the same goals as you, and the same time commitment as you. You could find, for example, a 3-night Mythic guild on Wowprogress or RaiderIO or a Recruitment Discord or Forum.
But I’m guessing that isn’t what you’re aiming for,
There are less ambitious guilds that do Normal raids.
Raids are about the only thing you actually need a guild for, and you don’t really need a guild for Normal or casual Heroic, since there will be PuGs (Pick-up Groups) going regularly - if you can get into them.
Everything else in the game can be done solo, or in an ad-hoc group that you will get in the Group Finder.
Pug life is pretty hard, though, so if you want to be ambitious about your goals, a group in a Guild or Community or Discord is a better way to go.
Guilds die over time, as people just stop playing. I’m in a couple of dead guilds, but since I am now a very casual solo player, I have no need to seek out new guilds to raid with.
As a practical matter for the casual player, if you can find a Community or Discord with a set of active people who do more or less the same content you want to do regularly, that’s as good as a Guild for you.
You will not even undertsand what content you might want to do until you hit max level and play at max for a season or so.
If I was looking for a group to join as a newcomer, I would start with some Discords - perhaps the WoWNoob Discord to start, maybe WoWMadeEasy. These are places where you can lurk and watch the chat and ask some questions when you have the general idea.
Joining a random guild means nothing. It is very unlikely that anyone in the guild will even notice. Random guilds are like hotels. But if you choose a large one with a lot of active members, it is likely that you will meet some people for some dungeons.