By default the Mail app of Windows 8 adds the Sent from Windows Mail signature to your emails. This article explains how to create a custom one in a couple of easy steps.
Open the Mail app. Press the Windows key + C to open the Charms bar. Click on the Settings Charms > Options.
Click on Account and select the mail account (if multiple) for which you want to edit your signature. The mail app allows you to create a different signature for each of the apps you manage. Toggle the Use an email signature to Yes:
Clear the Sent from Windows Mail signature and write a new one. You can, for example, add your contact details (address, phone number....).