You must complete the tutorial and be at least level 1 to join or create Clans.
When you've done all of that, you can open the clan menu by tapping on the Clan button on your lobby screen. There you can search for an existing clan by using the search bar or create your own clan.

Creating a clan:
1. Before creating a new clan, complete the tutorial;
2. Save up 5000 Gunbucks to pay for the creation of a new clan;
3. After this, go to the Clan tab, choose Create Clan, then type in the name for your clan, it's tag and description. Select an insignia, the Clan's type and a region; Then press the 5000 Gunbucks button.
4. Once you created the clan, you will automatically become the leader of this clan;
5. By creating a clan or joining a clan for the first time you will complete the main Join a Clan quest and will be awarded with 10 Golds and one regular crate. Congrats!