Like some of the replies posted above, it can be very hard for someone not naturally extroverted to come up with skills and talents, ie to be able to 'sell yourself' in an interview situation. But what you need to bear in mind is that everyone is unique and even something that may be seem totally natural or even mundane to you will not be to someone else and may be things that sets you apart.
So I was fortunate to grow up in an international environment and speak several languages as a result. I find that this has always helped me in communicating well with people from various backgrounds , provided me with an open mind and natural curiosity, being a good listener and empathetic person. It's also always a good idea to share thoughts with your close environment: ask your friends, colleagues, parents what they perceive your key strengths (or things that set you apart) are. You might be surprised and get something out of it that you didn't even think about ! Good luck.