Staying eternally youthful is trivially easy, you'd just have to use it less than 5 times a year to stay in your 20s. So I think that one's a given. Money is also not likely to be an issue considering you'd be better than any investment bank if you used your power just 30 times a year or so - just take out a loan every now and then and use it to pump yourself full of money if you're running short.
So we'll move on to making the world better. Going Death Note is possible, but a bad idea. Firstly due to the reasons that Death Note itself presents, with it being explicitly intended as a criticism of people who think that the world would be better if only we would arbitrarily kill certain people. I'd also say it's highly likely that someone else would take over if you killed one individual in a bad organization, or that if you killed an entire organization there would be power vacuum problems.
Changing fundamental rules of physics seems very dangerous. For instance, screwing with the oxidization energy of Carbon is likely to kill all living things due to their dependence on respiration. Changing any fundamental force would likely be even worse and would cause the galaxy or matter itself to disintegrate.
I think the better approach is to help boost the solutions rather than attacking the problem. For instance, taking 1% chunks out of HIV will help people, but will never eliminate the problem as your "1% less" gets less effective every time. Making the drugs 1% more effective, or 1% more available, or increasing the research speed for new drugs against viruses in general by 1% every day for a year or so though would dramatically boost elimination efforts, and would keep getting better every time you used it. If you got drug research up to, say, 10,000% of what it is now while also doing similar things to vaccine and drug effectiveness that means you'd also likely be free to address other problems, as new pathogens would likely be stamped out very rapidly by the incredibly quick drug responses. I think this is the most efficient and sustainable way to fight the world's ills.