Click on the icon on the upper right which is your personal settings.  Click on the "Settings" link to customize your settings.

select "My Email Settings" from the many options available to you in personal settings.

Here you can set your email name, email address (for replies), and add a Signature. These settings will apply to any emails you send directly from Salesforce to a contact.   (Note: this does not affect automated emails that are sent via workflows or processes)

Under your  'name' menu  in the header, select "My Settings"

Select My Email Settings from the left menu: