To enable two factor authentication click on your username at the top and select "Profile".
Under your profile you should be able to turn on two factor authentication by simply checking the "Enable two factor authentication" checkbox.
Google Authenticator is our recommended method for two factor authentication. The other available methods are SMS messages and Email messages. Both of them rely on different factors like your phone reception or the speed you'll get the email message.
Using Google Authenticator is immediate, it's a program that shows you a 6 digit number it calculates using the current time and a secret key you shared with it when you set it up the first time. That way we can identify it's really you even if your phone is not connected to the internet.
You can download Google Authenticator here: Android / iOS.
Once you have the program installed on your desired device click on your username at the top right of the screen and select "Google Authenticator" for the setup wizard. Once the setup is done, the next time you login you'll need to copy the code from the Google Authenticator app to get access to your account.