It depends on the your email program. Typically, in the "To" box, you type the e-mails separated by a comma (separated by a semi-colon in Microsoft Outlook).
For long lists of email where the people don't know each other and already have each other's email addresses, you should put the emails in the "bcc" (blind copy) field and put your own address in the "To" field. This will hide the list of recipients in the email. But only use "bcc" in this case, don't use bcc to let someone see an email you've written to someone without the person you are writing knowing that you've shared that email with the third party. That is unethical.