That is a question most people have great difficulty to answer. Some people have particuliar passion and a predisposition to succeed in what they are passionate, but these people are exceptions. For others, like me, we have several areas of interest, although this is not necessarily passions, but we have not yet developed our talents and skills to successfully move to one of these fields of interest.
This seems a lost cause, but this is not the case. Simply, we must take the path of experimentation to help us better define what we want to do and what we're gone do to achieve it. Moreover, we must add the chances of life, encounters with people with positif impact in your life, they will guide you, add to this, life events forcing you to make some difficult choices, forcing major changes in your life.
Whatever happens in your life, the most important thing is to learn to see life as it is and the way your act to it. You must believe in your potential, but you must learn to set goals large enough to force you out of your comfort zone. Take a look back at your life and you will find that whenever you learn something, you have developed new knowledge and new skills, it was situations in which you were uncomfortable, requiring from you an extra effort to get through the situation.
The more you dare try, the more you discover your talent and the better are the chances of discovering what you will do in your life. The secret is an action and build trust in yourself, but don't hurry, take time to get there.