When in SUmail, you can set up or change your Signature by going to Options --> See All Options --> Settings
Now select the "Mail" tab if needed (usually the default tab). You will see a section called "Email Signature." Enter the text for your signature in the box provided and use the toolbar to customize the signature by changing the font, text color, bolding the text, inserting a hyperlink, etc. You can't include an image in your signature in the Outlook Web App (SUmail web interface).
Be sure to click "Save" in the bottom-right hand corner when you are done.
If you want your signature to appear on all outgoing items (including replies and forwards), check the box for "Automatically include my signature on messages I send." If you don't check this box, you can include your signature on individual emails by going to the message toolbar in a new email message and selecting "Insert Signature."