To add a signature to all outgoing emails - click View settings on the View ribbon tab - from here you can click Accounts > Signatures (as shown below) to create/modify your signature settings

 

 

You can then click New signature to create a new signature.

 

You can enter a name for the signature.

 

You can then enter your signature text in the box below and format it as required

 

Now choose which account you wish to use this signature with, and select whether you want to add signatures automatically to new messages and replies/forwards

 

When you are done click Save to save the changes

 

 

Now when you create a new email message the signature will appear automatically as shown below - or if you have chosen not to add it automatically you can click the Signature button (as highlighted below) to insert your chosen signature