To delete your site, go on the Panel, click on the site you want to delete and then go on Site Settings.
Here you will find the Dele Site button.
Click on it to delete your site.
Remember that if the site is the only one in your account (you don't have other sites), by deleting it you will delete your account too.
Remember also that if your site is shared with other users and you decide to delete it, it will be deleted for all the users. If you just want to remove it from your account, not to delete it, click on the Remove from my account button.
Remember that deleting your site is an irreversible action.