Even after attaining Enlightenment, Lord Buddha spent his time lonely without teaching any of his wisdom and knowledge. The legend said that Lord Buddha was approached by many souls of human beings, as well as animals so that they could lead Lord Buddha to teach his omnipotent wisdoms and knowledge. The Lord Buddha decided to proclaim the Dharma that he just realized in the Bodh Gaya and wanted to entrust this doctrines to five ascetics who followed Lord Buddha i.e. five ascetics lead by Kaundinya. At that time, these five ascetics were staying in Isipatana i.e. Sarnath in present time. Then at the deer park, Lord Buddha met these five ascetics and thus began Lord Buddha's first preaching about Dhamma.
In the Deer Park, Lord Buddha addressed disappointed five ascetics
'Monks, these two extremes ought not to be practiced by one who has gone forth from the household life. (What are the two?) There is addiction to indulgence of sense-pleasures, which is low, coarse, the way of ordinary people, unworthy, and unprofitable; and there is addiction to self-mortification, which is painful, unworthy and unprofitable.'
After hearing these words, Kaundinya and four other ascetics realized that Lord Buddha had attained highest level of knowledge and started to listen Buddha first teaching. It is also said that many Gods and angels were also present during Lord Buddha preaching but Kaundinya and other four were the first human beings who listened Lord Buddha first teachings.
Then Lord Buddha started to teach the five ascetics about the Four NobleTruths i.e. the truth of suffering, cause of suffering, end of the suffering, and the way to end the suffering.
"The world is full of pain and suffering; the cause of suffering is craving or desire; the end of suffering is possible through Nirvana; the way to achieve Nirvana is through Middle Way." – Lord Buddha
See More: Noble Eightfold Path