All you have to do is go to 'account' and click 'leave club'. You then probably have to enter your password.
If you want to start again at a lower league club, then you have to do something different. I *think* you can go to the team page of a bot team and join that club, taking three players with you. If you're leaving totally, then you can just go. If someone wants to take over, then you need to arrange a time to do this so noone else can swoop in.
You'll be missed.