To compose a message, begin by clicking the "Compose" button on the toolbar near the top of the screen:
The compose message screen is split into four main sections. At the top, you will be able to enter the recipient and subject for your new email:
Alternatively, you can select a contact from the left pane to have it automatically populate the "To" field from your Address Book contacts:
If you'd like to attach some files to your email, the right pane will allow you to upload and attach files from your local file system:
Lastly, the large text area at the bottom of the screen provides an area for you to write your message:
When you're ready to send your message, click the "Send" button on the toolbar near the top of the screen. Alternatively, you can click the "Save" button to save your message as a draft to be sent at a later time.