As a minimum, you need to type the sender’s email address (usually either your address or the address of the mailing list/autoresponder) in the “From” text box, and the message subject in the text box labeled “Subject”.
Hint: You may be wondering how to include the name of the sender in the “From” text box. You can do it by surrounding the name with quotation marks.
For example:
“Your Name” <youraddress@yourdomain.com>
To insert a data field into the subject at the current location of the text-editing caret, use the “Insert Field” button 
located to the right of the “Subject” box.
Note: You can use data fields in other text boxes by typing them by hand. The reason only the “Subject” text box has a special button next to it is purely functional. This is the place, apart from the actual message text, you are going to use data fields most frequently. The “Insert Field” button for the message text box is located on the toolbar right above the text box.
If you are sending a message to your subscribers, you also need to click the “To” button, and select one or more target mailing lists. Read this to learn more: Tutorial: Sending Message to Subscribers.
If you want replies to this message to go to a certain address, type it in the “Reply-To” text box. If this box is empty, all replies go to the email address in the “From” text box (except for certain, usually delivery-status-related, messages that go to the “Return-Path” email address if it is defined).
You can use the “CC” text box to send copies to an email address. A copy is sent for every recipient the message is sent to. For instance, if you are sending a message to 1000 recipients with your email address in the “CC” text box, you are going to receive 1000 copies. The same applies to replies, automatic follow-ups etc. This text box is useful for testing purposes though.