I'm going to be the sucky do-gooder philanthropist here (after I buy myself an ancient castle or manor house, well away from people, and hide away in obscurity).
Considering that Elon Musk, the richest person on the planet, is only worth a paltry $203 billion and I have 20 TRILLION at my disposal, I would make sure that every single person on the planet has the essential things they need to live a decent life - food, clean water, housing, mental supports, a way to make an income. And I would start changing the way we run this planet, starting with the environment. I sure as hell wouldn't use my money to further space exploration until we've got a good handle on our own world.