Magento has dozens of built in email templates for you to use with your online store. To customize the content in them, follow these steps.
Go to System → Transactional Emails
Click Add New Template
From the Template dropdown menu, select the template you wish to edit and click Load Template.
Edit the template as needed.
Be sure to name the new template before you save it!
Your new template now appears in the list of templates under Transactional Emails
Change any settings associated with this email so that the new template is used and not the default. These settings are found under System → Configuration → Sales → Sales Emails