So, I always saw a cute girl sitting alone... But Im too shy to say hey! Im a shy guy :( If we don't know her, what topic do I have to talk to her?

Will you could just tell her that she looks nice or just try to say hi it will be hard but it will make you feel better once you do it I hope I helped

One of my guy friends would go up to her and say, "Hi, cute-girl-sitting-by-herself. May I sit next to you? My name is Bob. What's yours?"