Alliance unlocks when you reach rank PV2 (Level 10). You can then go to Main Menu, look for the bottom right button that says  ‘Alliance’, and click on it. This will take you to a new screen where you will see two buttons: Search Alliance (this is to join an existing Alliance) and Create Alliance. Select your preferred option and follow the steps according to your preference. I hope this helps.

[Creating a new Alliance]


[Join an Alliance]

Join an Alliance