I hate questions about email because nobody ever remembers to tell us what system they're using!
Are you using an email client (such as Outlook Express, Thunderbird or Windows Live Mail) or a web-based method (such as Outlook.com or Gmail.com)? Without knowing what you see on your screen it's hard to advise you.
However, in general, you can either create an email group (and then send one email to the whole of the group) or enter lots of different addresses into the 'To' field (or, preferably, the 'Bcc' field). Exactly how you do either of those will depend upon which email system you're using. This gives you the basic routine for use with email clients
http://www.bleepingcomputer.com/forums/t/43181/how-to-email-to-multiple-addresses-at-the-same-time/
but you might need to enter the addresses via a different method if you're using a web-based system.
You also need to remember that email providers usually limit the number of addresses you can send a single mailing to. (It's an anti-spam measure). The limit is often around 50 addresses but, with some email providers, it can be as low as 20, or even 10. So you should seek out that information (on your email provider's website or by asking here) before trying to send a single email to a large number of people at once.