When you hit the Write button, it will open a blank email to no one and in the To: field, you can type in an email address. If you’re sending to someone already in your address book, begin typing their name and the name/address should automatically fill in. This works the same for groups also. If you’re not sure about a name or address, you can hit the Contacts button and it will bring up a sidebar with your contacts to choose from.
If you want to CC: additional people, you can change the To: field to a CC:, BCC:, Reply-To:, Newsgroup:, or Followup-To:.
If you need to attach a document to the email, hit the Attach button, it will open a Finder/Explorer window and you can find your document and attach it.
If you decide not to send the email when you finish typing it, you can hit the Save button and the email will be saved in the Drafts folder for you to send later.