Figure out what kind of guild you want. Hard core? Social? Somewhere in the middle?
Figure out what you want to do. Lead raids? Stick to raiding and let someone else do raids, while you do guild stuff?
Figure out how much time you want to spend on administrivia. Guess how much time you'll spend dealing with crap. Then double it.
Figure out what your guild structure is going to be -- class officers? Just an officer cadre? Loot council?
Figure out how you're going to organize. Get a website. Get a vent server. I hosted and wrote my own guild management application and got vent service from pure-voice.net or something; there are a bunch of different vent providers. It'll cost you like $20/mo or something. You could hack together a livejournal and Google Calendar to do all your guild stuff if you really wanted to. It's not terribly difficult if you have folks with skills.
Leading a guild is a lot of work. It will take away from the time you have to actually play the game. Make sure it's what you want to do before you start doing it.
In my case, I didn't have to start from ground zero; it was a guild that reformed when the GM needed to take a break. We reorganized very quickly and barely missed a step in our progression. This is back in the 40 man raid days, so the numbers of people I was dealing with are very different from today's guilds; I imagine it's considerably easier nowadays.
If I were to do it again today, I would have made some personel changes. I would have retooled things a bit so raiding was less serious business and more fun ... after all, this is a game -- and if you're not enjoying yourself, why bother?
Some folks want to raid and treat the getting of purples with the seriousness of life and death. If that's the kind of guild you want to run, good luck to you.
But I think in a couple years you'll have more to show for it if you focus on having fun with a tightknit group of folks who happen to kill internet dragons together, instead of running some boiler room purple making factory.
But that's just me.
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