Open Outlook from withing your Applications folder.
In the top left of your screen select Outlook and then select Preferences... from the menu.
Select Signatures from the Outlook Preferences menu.
Click into the Signature box and begin typing your signature.
Right-click (or control-click) in the signature box and select Font... to change font, color, and size.
Click Format in the top menu and select Insert Picture to add a picture or logo.
Under Choose default signature, make sure your signature is selected under New Messages and Replies/Forwards.
To save just close the window. Outlook will automatically save the signature.