Here's a few quick pointers:
1). If you're new to WoW then it doesn't matter what guild you join, at first. You will want to join a Lv25 one ASAP for its Perks (such as +10% experience, +10% (or is that 20?) mount speed, etc.
2). Once you get to Level 90, then you need to make up your mind what exactly you want to do -- PvP, PvE Endgame, etc.
3). Any guild that asks you to join them is not going to be the ideal "endgame" guild if you want to do any "serious" raiding (currently Normal/Heroic Raids) or "serious" PvP (Rated Battlegrounds, Arena, etc). The guilds you want, are the guilds you need to apply to (and they'll check your gear/performance/etc). These guilds that you have to apply to will have better players in general. Usually, as you spend some time on any given server, you'll get to know who the "good" guilds are just by observing, listening to people, etc.
4). There are very little downsides to joining a guild, any guild, at low-level. Nobody's going to really ask you to do anything for them (because you aren't Lv90) unless you play a tank or a healer or some-such and someone else happens to be near the same level you are. You can always change guilds later (but you'll lose the guild reputation you built up, that's easy to get back though).
5). When joining a guild, visit the guild vendor (near the entrance in Stormwind for Alliance; near Grommash Hold in Orgrimmar for Horde) to see if that guild has some of the unlockables, such as Lil' Tyrecgosa (sp?), Armadillo pup, Dark Phoenix mount, etc. These can be bought (don't forget to take a couple gathering profs to get lots of money!) and used and will be available on any character on your account (except for faction-specific ones like the Alliance's Golden King mount), even if you leave the guild. These usually require Exalted with the guild.
Edited, Apr 21st 2014 7:13am by Lyrailis