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?

Then you will miss out on most of high school dating life, have 1000 regrets when you get to be a old man telling stories.

You either get the nerve to walk up and talk to them, or you know that you will not date for a few more years till you mature enough to not be scared.
** or end up 35 living at home with mom and dad still paying video games.