The core in being authentic, is to speak the truth. To be honest. But what does that really mean?
You need to be true to yourself. Really acknowledge your own emotions and why you feel them. Not to punish yourself, but to correct or simply learn from. Sometimes, it's even just to experience, to live. To expand your consciousness and embrace a fuller capability.
Then, be true to others, not only about how you feel, but what you need and why. Often we either get into the habit of complaining, to hide our own self-confidence, to not come off as a self-conceited snob, or we pretend that all is well, to not burden others, but really are denying ourselves the support we could get.
The deceit, however, often starts in things less personal. For some reason, most people have a hard time sharing their real opinion, their likes and dislikes and why. Our opinion are often based on others', when we really should use our own personal experiences, as well as our education, to share our own perspective. But for us to be able to, we must first feel allowed and respected. Preferably invited to.
So, now that you have learned to be grateful, let's look at the opposite and list what you lack. How can you instead refill yourself? Can you do something about this on your own, or do you need someone else to? Who, what and why? 'tis the reality!