If I took over the world, it would be through a mix of social engineering and outright total warfare.
The first thing I'd do is put out a personal ad ("President of the World"); I'd get responses easily no? Seriously though, the first thing I would do is use Switzerland as my base of operations, and redraw territorial boundaries (individual national leaders answer to me). Jerusalem would be an autonomous state similar to the Vatican; the Israelis and Palestinians can theoretically coexist in a state, contrary to what the Old Testament preaches. Sub-saharan Africa will be tricky though as there are far too many ethnic divisions to create a real state without a series of debt relief, job training, etc. programs. I shall actually meet with the NAACP, and seek to actively discredit the Nation of Islam, KKK, Aryan Nation, etc.
Divert funds from big tobacco, oil subsidies, etc. and put them to researching alternate energy sources. Dismantle nuclear weapons to use for energy; research how to convert radiation to energy. Get rid of our government cheese, or better yet distribute it to areas that need it more. Since I own the world, I can control import duties and make the cheese affordable for the starving masses. I would raise income taxes (a progressive tax) and lower sales taxes (a flat tax). Eliminate the regressive tax outright by not asking "what percentage of income was spent on what?" Privatize social security; have the government offer classes on how to best invest money.
Humanity's health is a very important concern; therefore I shall actually distribute condoms among Africa to help slow AIDS down (as opposed to Bush's administration which decries such an action as immoral). I shall fund research into cloning, genetic manipulation, etc. Irridating crops/pesticides/etc. are health hazards, so switching to other methods (drip irrigation, pest-eating spiders, etc) is another issue. Contrary to what most people in the organic movement believe, I do believe in genetically modified crops: imagine if our spinach were LOADED with protein that we'd all be like Popeye the Sailor Man. I believe in clinical immortality and would fund research into cold storage, longetivity vaccines, etc.
Space is also important; we need a way to protect our travellers from cosmic rays. One idea is the use of a water shield surrounding the craft. This would increase mass drastically unless an unusual method of water transportation was devised (like a space elevator). This means we need to reduce the costs to produce carbon nanotubes, perhaps our best bet for an elevator cable with the tensile strength for such an endeavor. I would then convert the ISS into a counterweight for this elevator, as well as a shipyard for spaceships. As it becomes more practical, we'll expand this shipyard and increase the number of elevators appropriately.
I believe in civil liberties; I believe consensual crimes aren't crimes. I believe in the legalization of marijuana, prostitution, etc. assuming it's not forced upon. Meaning a pimp (that kidnaps a girl and shoots her with crack so she becomes dependent on him so she sells her body to others for her next fix) is scum and deserves death. I don't believe in arresting people because they rented a motel for "immoral purposes," (boyfriend&girlfriend after prom night?) or for having an interest in kinky toys.
Who here would have me for world leader?