I would reduce the deforestation and help people get more practical education especially in poor and violent areas where kids are forced to take jobs because they don't get proper education, practical education in the sense making them learn about Cryptcurrency and Bitcoins, getting them hands-on internship etc, I think education for knowledge to earn money is way out of poverty which is one of the black-mark in our society, would love to get your opinions and thoughts on how you would impact the world when you are a billionaire.
I have some knowledge and training in biochemistry so I'd probably open my own lab and conduct scientific experiments. Having a billion dollars or even a few million dollars would free me from having to submit grant proposals and relieve me of any teaching responsibilities that usually come with becoming a researcher at a college or university.
If anything good comes out of my research, I'd probably put it into the public domain just like the way Jonas Salk did with his polio vaccine.