How can I have a positive outlook in life and not be be negative about most things?

Positive psychology. Choosing to have it as a focus, making a commitment to it, and establishing clear objective goals to get there - happiness is not often an articulated goal we consciously identify nor strive for. Savoring the moment, cultivating optimism, preferring flow experiences, engaging your faith, active socialization with many friends, resisting social comparison, exercise, practicing gratitude, forgiving.
Depression. Look into martin seligman's book learned optimism to try to shift your negative outlook.
Smile. You make the choices in your life. Buy books about lifestyle change. Read about inspirational figures. Smile. Really, smile more.