You may wish to include a portion of text such as your name, organization and contact details in each message you send. To do this, you can setup an email signature.
To setup an email signature, first click the Options button and select See All Options... from the menu.
Click Settings then click Mail. In the section titled E-Mail Signature, you can compose the text to use as a signature, and choose to automatically include it on messages your send. When you are happy with the text, ensure you click Save to keep your changes.