Don't go near anything you no little or nothing about. I suspect you are running into trouble because you may not know enough about Global Warming. It is an immensely complicated subject made worse by those who could tell us the truth but don't. And billions have been spent on it with very little certainty of what the truth really is.
Even greed and death have a lot to them that no one knows about unless they are confronting them. Death is especially tricky.
Do you go to school? I mean a brick building? What do you think about that? If you don't, what about the loneliness of home schooling?
What do you know about? What problem have you encountered that you understand? Boyfriends/girlfriends? Parents? Friends? What? When you can answer that then think about your characters. You won't have any trouble at all.