In order to prevent the drastic effects of global warming, we need to reduce greenhouse gas emissions, and limit deforestation.
However, there is only so much that a single person can do. The most important thing that you can do is to select government leaders who share these same goals. A single person can, for example switch from incandescent bulbs to fluorescent ones, but a government is able to limit tax breaks for the gas and coal industries, and use this money to invest in cleaner technologies.