First you'll want to have some idea what you want your guild to be. Guilds evolve over time, but it's nice to have some sort of idea so you can tell people who ask when you're getting started. What's the focus of the guild going to be? Raiding (casual or progression), PvP, do you just want it to be a social guild, or just a leveling guild? Some random other things:
It takes a long time to make a decent guild. Though you're probably aware of that if you helped raise one to 25. 6-8 months to get from where you started to something resembling what you wanted wouldn't be unusual. Be sure you're willing to invest the time.
Don't expect to get a lot of assistance. There's some helpful people out there, however most of them will just raid the guild bank, complain that you aren't doing any rated battlegrounds (or something) and leave after a few days. If people wanted to spam chat and help grow a guild, they'd start one of their own. Your banker got hacked and stole from the guild bank? Well Blizzard will usually be nice and return your stuff, but it's your 2 hours spent restocking and reorganizing things.
That being said recruitment will be hard in the beginning, even more so now that some people will see you're a low level guild, with few members and simply not want to join. Inviting everyone and anyone in the beginning certainly works, but many others find it annoying. If mass invites aren't your thing put together a guild summary/recruitment macro that you can spam in /1 as you go about your usual routine. If you can make it humorous or somehow interesting you'll get more bites.
Eventually as you get more established as a guild you can do things like put together an application, or be more selective somehow with recruits. This will mean fewer people joining, but you'll have a better turnover rate, and they'll usually (usually) be less trouble. Unfortunately these things can be tough to have when starting a guild. You need to have something worth them filling out an application for.
As for keeping people around, that's a bit tricky. It's politics, kinda. One thing you can do is help people make the guild their own. You can delegate things to the more responsible members. Guild banker, recruitment, PvP, raid leader, website admin, nanny (seriously solving in-guild issues is a real talent), etc. are some ideas of jobs. It can be a nice touch to have someone who comes up with fun things for the guild to do on occasion (naked gnome races anyone?).
More work isn't for everyone though. For others it is about the social environment. If you are a social/friendly person it'll help.
Give the responsible people a voice. I did an officer vote on a lot of issues affecting the guild.
Finally many people will just leave for many different reasons. Don't worry too much about it, it happens. If a problem person leaves, good riddance. If someone good leaves, let them know the door is always open if they want to come back. Don't burn the bridge, good guildies are hard to find. We had people who didn't fit in with our guild, but were quite happy to join us for the occasional herioc, or fill a raid spot.
Okay, wow that's a massive block of text. Sorry about that, I was on a nostalgic rambling. Here's a summary...
TL:DR = Work hard; the more you put into it, the more you'll get out of it.
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That monster in the mirror, he just might be you. -Grover