Well, isn't that just the biggest question. Where to start? The beginning was probably best, so Raven started there.
It wasn't a short tale, but she told Kurt about how when she was born her parents were horrified by her appearance and abandoned her; how she survived on her own for her beginning years, using her chameleon powers to get herself food and shelter; about the night she met Charles and he changed her life by being the first person to see her true form and not run away.
She told him about the years she spent at Charles' side, about learning to hide who she was. She paused, however, when she reached the point where Agent McTaggart came into their lives and changed everything. That was less her story and more the story of mutants as a whole.