Answer:
(Buddha taught the Three Universal Truths and The Four Noble Truths for 45 years)
Three Universal Truths
Everything in life is impermanent (always changing), a life based on possessing things doesn't make you happy (because nothing is permanent), there is no eternal, unchanging soul. "Self" is just a collection of changing characteristics or attributes.
Four Noble Truths
Human life has a lot of suffering from our greed, and there is a way to end it, which is to follow the "Middle Path".
Tl;dr Suffering is because of human greed, life is always changing, and to get to Nirvana (no suffering) live righteously and meditate.