Identifying what you like is the 1st step.
The rest will come as you discuss and look at ways to learn more on the subject. Its also important that you are able to get different views and be able to also hear a point of view that is different from you. Passion is also a emotion you bring to the subject/topic. So the short answer is Listening is the 2nd step.
the 3rd step which is the continuous process is as you become an expert you will gain confidence and that also brings about passion as you will talk with conviction.