To add a signature which can be appended to messages you send, from the Outlook menu (in the Apple menu bar at the top of the screen) select Preferences. This will open the Preferences window:
Click on the Signature icon (middle icon of center row) which will open the Signatures window:
Type your signature in the right hand pane and make sure that the Standard option in the left hand pane is ticked. To add an alternative signature, click on the + icon at the bottom left hand corner of the pane which will create a new signature option which can be edited in the same way.
When composing an email message, click the Signatures icon in the message toolbar and then select the signature from the list displayed (usually just Standard unless you have added any additional ones):