Completing any of the steps in the security section will enable two-factor authentication. Adding any of these settings will require an additional step when a new device logs into your account.
You can access the security section by clicking on your name (upper right) and selecting the "Security" link. We highly recommend you add a phone number which will enable phone verification via text/SMS.
Please note that adding or changing a security feature or setting on one site will change it on all of our marketplaces.