The following points may help you:
1. The Nawab of Bengal Murshid Quli Khan,was appointed as the first Diwan of Bengal.
2. Bengal has emerged as important trading centre, the East India Company had started its trading activities from the banks of river Hoogly, which had developed as an important sea port.
3. During the reign of Aurangzeb, the company was granted concessions that allowed them to trade freely without any duties and that enabled them to expand their trade in Bengal.
4. After the death of Aurangzeb, the Nawabs of Bengal tried to assert their dominance and refused to grant company's concessions, the Nawabs rather wanted to check the expanding tendencies of the East India company and accused it of deceit.
5. However, because of the Company's refusal to pay taxes and levying heavy duties on Indian goods entering Bengal, and continuous fortification of Calcutta, led to the outbreak of Battle of Plassey in 1757, which resulted in company's victory.
6. The Nawabs who were the Diwan, did protest but because of the shrewdness of Robert Clive and and deceit by Mir Jafar who was promised the throne of Bengal by clive resulted in company's victory.
7. The nawabs installed by the Britsih like Mir Jafar, Mir quasim could not attack because of heavy demands , finances they paid to comapny to meet its own needs and also since they were puppet rulers.
8. It was after the battle of Buxor that the company was appointed Diwan which gave it right over trade in India and were granted exclusive farman to trade in India.
9. In 1765, the Company was made the Diwan , that is the chief financial administrator which administered the revenue and was given right to collect Diwani, that is the revenue.