If I ruled the world, I'd reform education. Those damn nationalists keep going for piss-poor education and fighting for their rights to get a job even when they don't qualify. I'd design it to give everyone an equal chance to work hard towards being qualified, but make education systems more brutal on weeding out graduates who perform poorly. Right now, you've got people with straight A's coming from bad universities and comp science graduates who can't even answer questions from the first chapter of C++ for Dummies.
It'd have to start from school, though. Western schools put too much focus on teaching kids to be creative (with SAT, ACT, etc), Eastern ones too much on rote learning. Kids should be at a level where all of them can know derivatives, complex numbers, and basic statistics comfortably, as well as some world history... on WHY things happened not WHAT happened. I'd put less focus on PE, art, literature, religion, and music, because it's all personal taste. I'd force them to take at least one of those subjects, to be cultural and stuff.
Education solves half the world's problems. Malaria? AIDS? Get more doctors and teach them proper health care. Poverty? Teach them to earn money, don't goddamn have million dollar campaigns to stop poor people from breeding. Dictatorships? Corruption? People are more reluctant to supress the rights of intellectuals.
Ruling the world means that everyone in the world's the citizen of the same country, not this silly my country vs yours crap. Everyone gets their country built to an average level of development, though each country/state still gets to keep a significant portion of the wealth they produce, otherwise the nationalists will still get angry.
Once we get education and economy right, we can focus on the important stuff, like military and conquering other planets.