How do I talk to a girl who I've never talk before?

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Same way you start a conversation with anyone else. Starting a conversation with someone because you find them attractive and want to get to know them more is a perfectly good reason to start a conversation with them. Find a way to open and then nurture the thread of communication until it maintains itself.

Keep talking until real life starts commanding your attention, then ask if she wants to continue the conversation, perhaps over dinner.