Go to Preferences > Accounts > Signature to change or add signatures to your outgoing email messages. Signatures are created on a per-account basis, so you can configure each email account with multiple signatures.
Additionally, you can control whether a signature is automatically added to a message (again, on a per-account basis). Just go to Preferences > Accounts > (select an account) then click on 'Composer' tab. Choose whether to add a signature to the message while replying.
You will notice in the above picture that you can also include signatures in Quick Replies.