A ringtone you loaded or a preloaded ringtone? Preloaded ringtones cannot be deleted. Ringtones you have loaded, just go to the media app > ringtones, highlight the ringtone > press menu > Delete.
Also, when posting, please use a descriptive title for your post other than "Help" or "please help."
One of the standard built-in ringtones? You can't delete them, you have to reinstall the OS software without the ringtone. Unfortunately you can't deselect only one, it's all or nothing.
If this is a Custom ringtone you downloaded I'm not certain, but you should be able to find the file somewhere in the Media browser/explorer and delete it. (Media, look for the Ringtone item and start exploring until you find it)
