Empower women.
I'm serious. If you want to curtail the out-of-control growth rate, and consensually so, women need to be able to choose birth control; they need to not fear rape; they need to not be viewed as an economic liability (where e.g. parents with a son might stop there but parents with a daughter say "hey, we need another child so that someone cares for us in our old age").
The places with high birth rates will be the places where birth control is either hard to find or culturally discouraged.
To test this, I went to the CIA World Factbook and multiplied their population growth rates (which are sadly from 2002) against populations to find that the situation looks approximately like this:
Country Millions added/yr
India 15.79
China 6.45
Nigeria 4.34
Ethiopia 2.98
Pakistan 2.95
United States 2.82
Indonesia 2.58
Bangladesh 2.55
Brazil 2.26
Philippines 1.94
Congo, D.R. of 1.90
Egypt 1.61
Uganda 1.28
Mexico 1.25
Kenya 1.05
Iran 0.99
Vietnam 0.96
Turkey 0.96
Tanzania 0.85
Iraq 0.73
The net is 76.46, summing over all countries, so India is 21% of the problem, China is 8.4% of it. One nice thing about this metric is that both Bangladesh and China are reasonably high here -- Bangladesh of course is struggling with some of the highest population density in the world, while China has a low growth rate but their population is absolutely staggering. It's very hard to get them both to appear in a top 10 list of "where the problem is".
If past history is any indication, China will self-regulate its population by descending into civil war, so the major thing to focus on is India + Pakistan. However, if you could make a dent in the United States somehow, that could also help to reduce our population-deficit.