Be default the signature added to emails will be "Sent from my iPhone/iPad" but this can easily be amended.

1. From the home screen go to Settings.
2. Scroll down the left menu and tap on Mail, Contacts, Calendars.
3. Now scroll down the right menu and tap on Signature, at the bottom of the MAIL section.
4. You'll see the current signature on the right. Tap on it and the keyboard will pop up, allowing you to amend the signature.
5. When finished, return to the setting menu using the blue link at the top right. The changes will be saved automatically.

Your signature has now been changed.