If you are talking about entering a password for every time you open the Mail app, I'm afraid it's not possible on the stock iPad. Apple has not made it so that you can password-protect individual apps. Sorry.
Only possible i.e. on the GMail-App (and may as well be in other 3rd party apps), but not in the native mail-app.
Apple's idea is that an i-device is a personal device and not meant for sharing.
