You can easily add a signature to your Emails. Go to Setup -> Personal Settings -> Account Information. You may need to scroll down a little bit to reveal the Signature. Click Edit to the Right of Signature. You can create a signature using the editor, or you can add an image of your signature by clicking on the Add image icon.
To add your signature to your outgoing emails, create an email template (see earlier Tip on how to create a template) and click the Add User Signature Checkbox in the Edit screen.