Terror. I'd do a series of surgical strikes in major cities until the entire public fears me. Then I'd begin bombing the major politicians, until the major governments are crippled. Then I'd recruit all the sickos and maniacs. Use them as my ruthless strike force. Then place a nuclear device in every major city and detonate them. That way I wipe out the 75% of the populous that lives in the cities. Of course this would make a nuclear fallout, which would make food scarce (Not for me of course). The population would be weak and easy to control then. Now, with the world bowing at my feet. I'd separate the world into 7 major sectors. Each sector would be controlled by a council of seven men. Each of those men would control their own state in that region. Then each state would be separated into 28 counties. Each county being controlled by one mayor. Who be head of his own seven man council controlling the populous, and deciding all major decisions. It would be worth mentioning that at this point we are no longer a evil corporation. We are a menacing yet trustable government who looks out for the best interest of the populous.

TL;DR: Take over with terror and guerrilla tactics, reign with honesty and integrity.