The first teaching that the Buddha gave after attaining complete and perfect awakening was on the Four Noble Truths. The importance of this teaching is that the Buddha laid a framework for all the rest of his teachings, so that at any point a practitioner on the path knows how to relate the teachings back to their own life and experience.
First is the truth of suffering, that no matter where we look in the world suffering is always just around the corner, even if we have gained enormous wealth, fame or power.
Second is the truth of origin, it is because we invest in conditioned existence that this is the case. It is because we invest in things outside of ourselves, things that are of a temporary nature, that happiness continues to evade us.
Third is the truth of cessation, that it is indeed possible to find lasting happiness, a genuine inner peace and confidence in one’s own nature.
Fourth is the truth of the path, that by learning, contemplating and putting these teachings into practice we can indeed arrive at that state.
We can compare this to the metaphor of a sick person. If someone is sick, they go to a doctor to get a diagnosis. The doctor says ‘this is the cause of your sickness, and it is possible to be free from this sickness but you need to take this medicine’.