First I make myself a fact of time and space (like Captain Jack Harkness). This way I won't get killed by some dumbass political machine wanting to maintain power. Then I would rid the world of hunger, disease, want of power, bloodlust, hatred and all other things that lead to war. Then I would provide all peoples the knowledge that all peoples really are equal and beautiful, so no one would have any more reasons to fight and harm each other. Then I would rid the world of lameness, make it a biological fact that all peoples live until the age of 80 then keel over dead (barring accidents before 80), and create a perfect contraceptive to allow you to have sex infinitely without pregnancy until you want it. Then I will make it so soulmates are a real thing.
Then I'm going to announce to the world what I have done. They will be happy, free, and content. Then eventually the sun will go super nova or some shit so I'll be finding us a new planet, but hey I've got infinite knowledge so....