You can create a Tribe by going to the Map in game and tapping on Tribal Island. The first window that pops up will be invites that you've received from other players, but if you want to start a brand new Tribe, you'll want to click on the tab at the top of the menu called "My Tribe"

A new menu will appear with the option to either Create or Join a Tribe. Tap on 'Create', it will ask you to name your Tribe and after that you can choose what Monster you want on the Island. Your very own Tribe is now created! Now you can invite your friends to join so you can unlock the whole song!