To join a Clan, first go to the Clans section of the game by tapping the Clans icon from the flock screen. Then, tap "Join a Clan!" You'll now see a list of Clans that you can scroll through. If you see a Clan that interests you, simply tap where it says "Join!" on that Clan!
You can also use the "Search Clans" feature to search for a specific Clan by name or ID#, or you can do a filtered search by Clan Location, Clan Type, and number of Clan Members. This will then give you a list of Clans that meet those search criteria.