Anyone can create a clan. Just press the G key while in-game to set one up. You can invite others to your clan just by being on the same tile as them, and as of right now if you leave a clan you must wait 24 hours before joining another one, even though the in-game tool tip stills says you must wait 3 days (72 hours).
When making a clan, you are able to choose from a variety of colors and sigil combinations for your flags and wings. It is currently not possible to transfer leadership of a clan.
Since season 2 clans have been capped at 50 people, this is something to keep in mind when forming a clan. This is to prevent mega clans, even though mega clans just split into sister clans.