To create an Alliance in Battlecry, you need to be at least Level 30. Once created, you become the General of that Alliance.
If you want to join an Alliance instead, you must be at least Level 25. You can browse available Alliances and send a request to join one that suits you.
Your request can be accepted by the Alliance’s General, Major General, or Lieutenant General.
Joining an active and strong Alliance is usually recommended, as it gives you immediate access to more gameplay features and rewards.