You have to like yourself and accept yourself. It helps to realize that everyone has issues with confidence from time to time, and in different situations. Everyone questions themselves and wonders what they could be doing better.
What are you good at? What are you interested in? Focus on making yourself comfortable with the knowledge you have and expand on it. Remind yourself that nobody is perfect and you are not expected to be perfect. Just make sure you always give your best effort and always try to learn.