You can send request by choosing clan in rating or by finding clan that you want to enter in search. If you were invited to the clan, you will see notification when you enter the game:
If you press “Postpone” the window will close, but will be shown to you during three game enters. In 4th time there will be to opportunity to set aside.
If clan leader or its deputy has excluded you from the clan, you can join other clan straight away. If you quit clan by yourself, you would have to wait 14 days before entering another one.