My clan is strong, respected and has several allied neighbors. It seems like time to form a tribe, but I have no idea how (or if I'm even able).
So, how do you form a tribe, and what are the requirements for doing so?
Early in the game, the most important thing you can do is be successful, and flaunt it. Make friends with neighboring tribes. Establish caravans and trade routes. Lead successful raids. Complete some HeroQuests. Be generous to your allies, and gather them together to defeat otherwise insurmountable foes like monsters. Your goal here is to be perceived as kingly by other tribes - the Sacred Time recap screen will do a good job of showing how you're doing at this.
The other important requisite is the Making of the Storm Tribe HeroQuest. You must complete this one in order to have any chance at forming a tribe. Your recreation in the spirit world of the god Orlanth's formation of the first tribe is an important signal to neighboring clans that you know what the score is.
Once you've done that, eventually your advisors will begin to inform you that the signs and portents are aligning such that the time to form a tribe is coming. You'll begin receiving emissaries from neighboring clans, and a long negotiation process begins. Make enough promises, give enough gifts, and scratch enough backs, and you'll have a tribes moot. If you haven't made too many promises that aren't contradictory with each other, you might even be successful!