It isn't always easy to start a conversation with people you are just encountering. It takes practice but where do you start? Watching for tell tale signs of whether a person is approachable is a good place. If they are open, then a single comment on something we notice, like a book, can be a great ice breaker. Giving a compliment often makes a person smile and perhaps even make their day. 
Sometimes it might not work out well but when it does it feels so gratifying. Can even make our day to put a smile on another's face.