How did the king of Qin become the ruler of China's first centralised empire?

Qin Shi Huang, born as Ying Zheng in 259 BC, was the son of the king of the Qin State. At the age of thirteen, he succeeded his father's regality. Ying Zheng was very aggressive and ambitious at an early age. He assumed full power at 22 by ridding himself of his premier, Lu Buwei, who acted as regent while he was a minor. He wanted to unify and subjugate all the states like Han, Zhao, Wei, Chu, Yan and Qi by the powerful political, economic and military strength of the Qin State. Ying Zheng realized his ambition and built the first feudal and centralized empire in Chinese history in 221 BC. This was what we called - the Qin Dynasty (221 BC - 206 BC). Ying Zheng was the first emperor of a united China, so he proclaimed himself Qin Shi Huang.