The only way to take over the world is through peaceful means. Yeah, that's probably the gayest thing you've ever heard, but it's true. If there was only one nuclear power in the world, then that country could potentially take over the world through militaristic action, but as long as more than one country has nuclear weapons, world domination is impossible. As for those suggesting crippling the world's economy as a means to an end, all the economic subterfuge in the world won't disarm those warheads. You would need to create a peaceful alliance between all the countries in the world using a setup similar to the United States where every country in the world has one central government with its own laws and means of enforcing those laws, and anything not specifically determined by this global law would be up to each individual state (or in this case country) to decide. Once such a system is in place, you may attempt to become president of that organization, and from there just keep pushing the limits of what is considered to be within your power as much as you can, but with enough restraint that people don't realize you're slowly turning the world into a dictatorship.

...but I guess that's just how I'd do it. Johnny Walker and some rope's not a bad idea either.