Sharka's Guild Spreadsheet:
It costs 2000 Mesos to create a guild, and you can create a guild by opening your Guild panel (default is the G key) and creating a guild from there.
For guild missions/quests, check out the Guild Daily/Weekly page.
To be a good guild member, make sure to donate 100,000 mesos every day, check in, and do your daily/weekly guild missions. Doing so will contribute to helping your guild level up, get more members, and expand the guild house!
You will also have the ability to pay for personal buffs that last for an hour. These may be useful when you are prepared to do something important, like Dark Descent or a Chaos Raid.