At the top there is the From dropdown box, which allows you to set the address that the person receiving your mail will see in their From column when they view the email. Also when they click on their reply button, the response will be sent to the address you select here
In the To textbox, write the email address of the person you wish to send this message to. In the future, you'll be able to use aliases to the entries in your Address Book.
In the Subject textbox, write the subject of your email.
Below the Subject textbox is where you type the contents of your email message.
Under the body textbox, you'll see the Attachment box. If you wish to attach a file to your message, click the Browse... button and select the file you wish to attach. As of right now, we only allow one file attachment per message when composing in the webmail interface, but that will be changed in the near future
If you wish to add CC or BCC headers to your message, click on the More Options link to expose the appropriate textboxes. CC means 'carbon copy' and BCC means 'blind carbon copy'. Both of these allow you to specify a comma-separated list of email addresses that will receive this message. The important difference is that the addresses listed in the CC box will be visible to all recipients of the message, whereas the addresses listed in the BCC field will not be visible to anyone.