Authy
Authy is the leading 2 Factor service, with a great guide on how to add 2Factor to individual accounts, if you need step by step guide, check out here!
Recommended if you want to use multiple devices to get your codes, back it up incase you lose your device and more features!
Google Authenticator
Found on your market places (iOS app store or Play Store for Android) is a straight forward app with minimal features. If you get stuck or need help check out google's guide.
FreeOTP
An OpenSource alternative to Google Authenticator, very simple app with few features.
Recommended if you want an open source app that does not need features or want to sync across multiple devices and want something straight forward.