If you're using Outlook 2007 the Signature settings are under the Tools menu. Click Tools->Options and you'll get a popup window with some tabs in it.
Switch to the Mail Format tab and click on the "Signatures" button
You'll get a window that looks a lot like this:
Under "Choose the default signature" find the signature for your restaurant and set it to be the default for New messages and Replies/forwards. Please edit the signature to add your personal information in the editor box in the bottom. Just select the signature for your restaurant, edit it, and click save.
Outlook Signatures >