The East India company was founded in England in 1599 and soon began building trading warehouse in India.
At this time India was not a united country. There were many small states, each with it's own ruler. The East India company sometimes asked for permission to trade in Indian states but sometimes it did not.
At the same time, the French had their own East India company. Competition between the British and the French was fierce. In 1700 it escalated into a full scale war and the armies were dragged into the conflict.
The British agreed that they would help any Indian ruler who would help them trade and they would defeat any ruler who blocked their trade. In 1757 Robert Clive persuaded an Indian general to fight him with him and overthrow Siraj-ud-Duala the ruler of Bengal. The battle was won and Sirja-ud-Duala was captured and killed. This gave the British control of Bemngal.