Use Perform find the records for the contacts to which you want to send this email.
Select Send | Mail from the File menu.
One of your options is: "Multiple Emails, one for each member of the found set." You'll need to select fields in your database for at least the email address so that each person gets an email.
That's the basic idea. Things get more complex from there and it can all be done via a script so that you can just click a button to do it, but you'll first want to make sure you can do this "by hand" as the steps will be the same when you create a script for it.
Note: None of this works if you don't have a compatible Email client application or accesse to an SMTP mail server. Check filemaker help for details and a list of compatible client software for you operating system.