To add a signature to your Outlook go to the search bar at the top of Outlook and start typing "signature"
When the "Signature" result shows, click on it then click Signatures...
In the Signatures and Stationery menu you can create, edit and delete your signatures. Create your signature, give it a name and edit it in the Edit signature section. You can copy and paste from a previous email if you've had a signature before.
Once created, ensure that in the "New messages" and "Replies/forwards" drop downs, you have selected the signature otherwise it will not appear on new emails or replies/forwards.