How do I send e-mails without showing all other addresses?

OK, so never did this before, but have seen e-mail with "undisclosed recipients". So, I have numerous names to send a letter too, but don't want to show their e-mail address. I have gone into "mail" , but I don't know what it would be under. Well, Im old....and don't know this stuff. Please help. Some of the answers I looked up were still not very clear to this ole gal.