As an admin, if you want to have a common signature for all ticket replies, click on Admin > Workflows > Email notifications > Templates > Agent reply template > edit and set up a signature.
If you want to have a specific signature for each agent, then click on Admin > Team > Agents > Edit and then set a specific signature.
The agents can also set their own signature by clicking on the profile picture at the top right corner and then choosing 'Profile Settings'.