How should I approach a girl and what should I talk about?

It’s been many a times that I have seen some girl whom I wanted to talk to but I didn’t, probably because I am shy or anxious, or maybe even sacred how would she react. How should I approach her? How should I start a conversation? What should I talk about to make her interested? What kind of conversation or topics will make her interested? And yes how not to stalk? :(