How do I make myself into a more positive person?

I'm always thinking about death and how the world's going to end. I change moods easily. I show an upbeat person when I'm with my family than with my friends. I always make these promises to myself like, "tommorow I'm going to be a better person!" But, it never happens. I'm always the same boring and cynical person everyday. How can I change? How do I be a better person?