You can use the signature information to append information to every mail. The signature is added at the end of each email sent from your account. You can use the signature to include information like Phone numbers, addresses, etc. The information should be information, which remains common across mails.
In the 'Mail Preferences' page, select the 'Signature' field to modify your signature. The signature field is a text box into which you can add information. You can use the text box like any other field in which you enter information. You can format the position of the text. The signature in your mail will look the same as what is displayed in the signature field.