Rather than telling why you should look more confident? Let's have a conversation between you and Me. I will refer myself as 'Motivation With Kailas (MWK)':
You: How can I look more confident in a few minutes?
MWK: The first minute, regardless you are sitting or standing, do make your poster straight now. Initially, you may feel little stiffness, but you can take 2-3 deep breaths to be relaxed and straight.
You: Thanks, I am comfortable. How do I know that my posture is straight?