It's a fine day today. How should I reply to her?

1, when someone chats with you, you can echo the other person's words and say, That's great. I think the weather forecast says it will be sunny today, which seems to be true.

Generally, this is when two strangers have nothing to talk about before they start chatting. If you don't know how to answer it, you can smile back and change the topic to continue talking.