On your Tour find the Email Settings icon and click on it. Log in using your username/password for your @vzw.blackberry.net password. Don't have one? Create one, you'll need it for stuff like this.
From there you'll see a list of email accounts set up on your device, highlight the desired one and click on it and select Edit. Scroll down the list of options until you get to Signature. Edit the field as desired and save. Repeat as needed for your different email accounts.
From this screen you can also set up email forwarding, send your service books if you're having trouble sending/receiving emails, and create filters. So it's a good idea to become familiar with this screen.