You can join a clan in the following ways:
1) By accepting an invitaton from a clan leader, which you will be notified of through your ingame smartphone.
2) By applying for an open clan. To do this, open the clan menu , select a clan or look it up by the name, click on it and press "Apply for the clan".
The clan leader will receive a notification of your application and will approve or dissaprove it in his discretion.
Regards, Nexters Support Team