Being the sick puppy I am, I'd use the Deathnote to make the world my plaything. But first, I'd test it on an international figure and have them die in a manner that could only be possible with a Deathnote. For example, I'd have the Pope be hit by a small meteroite during his address to the people on Christmas day. Not because I have anything against him personally, but because then I'd know it had to be the Deathnote's doing. After that, I'd become the most sadistic prick this world has ever known. There's one idividual I've actually taken a bit of time to perfect how I'd have him die:
D**** T****, Compications due to AIDS
Arrested for assult in a bar. While in prison, he is repeatedly gang raped, contracting HIV from an inmate who did not know he was infected.
P.S. I actually have a cosplay Deathnote that I had autographed by Kirby Morrow, the voice actor who did Teru Mikami in the English dub (also the voice of Miroku on Inuyasha).