Being part of a guild can be a great experience. It gives you a pool of people that you can draw on for crafting, running flashpoints and operations, RP, and general hanging out. Being in a guild isn't *necessary*. You can level to 50 just fine without a guild. However, it's a nice additional benefit. Additionally, since there are servers with insufficient population, when you join a guild you're more likely to have other people around and it can make the server less lonely.
So how do you choose a guild? It's a daunting question. There are SO many options!
You have to make a few decisions before you can wisely start your search.
Are you looking for a guild first, or a server first?
The first decision to make is are you going to pick a server you like, and then find a guild there? If so, here's more information about how to pick a server (LINK HERE
Or you can find a guild you like and then join their server... read on!
Most importantly, consider what you WANT from a guild.
Regular events? Help with gear or powerlevelling? Hardcore ops and hard-mode flashpoints? Required attendance at raids? RP events? An exciting competitive atmosphere? A compelling RP story? Fun people to hang out with? Until you KNOW what you want, it's going to be difficult to FIND what you want. :-)
If you're so new that you don't really know what you want from a guild, that's okay too. You can either wait a while and play the game for maybe 8-12 hours, level a bit, and then go "shopping" for a guild. Alternately, you can try to find a guild before you even start the game. Both are valid ways to go about the process.
Once you've got these things narrowed down, the search can begin in earnest.
If you're already committed to a particular server or server type, then go find the forums for that server.
Click on the type of server you want. Of course, if you're looking for a PvE or PvP server, there are so many that they're broken down into alphabetical sections. If you click on an alphabetical section, then you'll see a new subforum called "Guild Recruitment and Looking for Guild". See who's recruiting and get the names and web addresses of likely guilds. Of course, that may mean looking through a lot of subfora.
If you're committed to one particular server, you've got a little easier time of it - click on your type of server, then your server alphabetical group, then look for your particular server.
Try the Ideal Guild Quiz
Fast, free, and you don't have to register. It's a series of questions that you answer to define your play style and what you want from a guild. The database compares them to the way guild leaders describe their guilds and matches you up. You can set the quiz to only look at a particular server or server type.
[suspicious link removed]
It's limited to those guilds who have *chosen* to be on it, but that's still a pretty good selection. It will give you the top three best matches for your playstyle.
Third, teh interwebz!
Google your server and see what guilds appear to be active. Some servers have external server-wide forums for all people on that server and these forums are not officially associated with SWTOR. This is a good way to get some information about the general climate of the server and to see which guilds are doing things with other guilds. Look for Facebook pages and Youtube videos or channels associated with your server.
Also, trying the big guild host websites can be useful:
Plus several other sites have guild links:
Fourth, do your homework once you've narrowed down your search to a few guilds.
I know, it doesn't sound fun, but a little homework now can save you a lot of wasted time so you don't join a guild, get situated, and then discover that you don't like it.
4A: Google stalk the guilds you're interested in.
Look for that guild name in the SWTOR forums and external forums. How do these people act online? Are you impressed? What's their website like? Is it paid for or does it have banner ads? Do they have a voice chat server? Do they have a cafepress store? Facebook page? Twitter feed? Youtube videos? Do they advertise guild events and are they open to non-guild people? Do they have events with other guilds such as PvP date night with an adversary guild?
4B: In-game stalk the guilds you're interested in.
If the guild runs open events, or participates in server-wide events, go to these events. Look for people with that guild tag and introduce yourself. Ask them what they think of their guild. Play against some of them in PvP. Do they cheat or trash talk? Do they play hard and fair? What are they like in general chat? Check out the GTN and see if you recognize their names as selling things in the auctions.
These all tell you something about that server and the guilds you're interested in. How you interpret that data is up to you, of course!