Go into "Setup" (folder with little green spoke symbol) and click on "Email Settings" then highlight the email account you want to edit, then hit the BlackBerry Menu button (black button with little white dots left of trackball), then select edit and it will bring up the screen where you can edit your signature. You can also edit your name there as well as a couple of other settings.
Some suggest not including you email address or other information such as other email addresses, phone numbers or street addresses there to avoid spam and such. I guess it depends upon how you use the email account.