On this page of the Diablo Immortal guide, you will learn about the requirements for creating your own clan.
The possibility of creating your own clan in Diablo Immortal will appear only after reaching level 30. However, if you do the main story quests, you will definitely reach this level quickly. You will also need 3,000 Platinum to create a clan in Diablo Immortal.
After reaching level 30 and collecting the appropriate amount of Platinum, go to the Adventurer tab, and then to the Clan tab. In this window, just select the "Create a Clan" option.
You will be able to select a clan name, as well as write a message that will be seen by every member. You can also choose your preferred language to make communication between clan members easier.