In order to send the same message to multiple recipients, you generally need to use the Address Book.
From your New Mail window, click on the Address Book button.

Once you are in the Address Book screen, you should scroll through the list of possible recipients and double click on each potential recipient.
As you double click on each person, they will be added to the list of people appearing underneath the To: button.

You can also send to multiple recipients by designating their message as a Carbon Copy (CC:) or Blind Copy (BC:).
To do this, click on the CC: or BC: button before double-clicking the name of your intended recipient(s).
The difference between a Carbon Copy and a Blind Copy is that a person designated to receive a Blind Copy won't know who else was sent the same message, whereas a Carbon Copy recipient can see who else was on the list of recipients.

The message with multiple recipients as well as carbon and blind copy recipients will look like this: