If you wish to add a signature to the bottom of your emails, where and how you set this depends on how you send emails.
If you use Webmail
you can add a signature in Settings > Identities > [click your address] > Signature. You can turn on 'HTML signature' if you want font/formatting options or to add an image. Once you have saved your signature, the signature button on the compose page will become active.
If you use email software on your computer/mobile device you should set the signature within your email software as per your software's instructions.