When and where was the first major agitation led by Gandhi, tell me who this agitation was against,  which crop led to this agitation and why this agitation happened.

Gandhi's first major achievement came in 1917 with the Champaran agitation in Bihar. The Champaran agitation pitted the local peasantry against largely Anglo-Indian plantation owners who were backed by the local administration. The peasants were forced to grow indigo (Indigofera sp.), a cash crop for Indigo dye whose demand had been declining over two decades, and were forced to sell their crops to the planters at a fixed price. Unhappy with this, the peasantry appealed to Gandhi at his ashram in Ahmedabad. Pursuing a strategy of nonviolent protest, Gandhi took the administration by surprise and won concessions from the authorities.

First major agitation was in 1917 in Champaran Bihar state. The agitation was against Anglo-Indian plantation owners. Indigo was the crop that led to this agitation and the reason was the lower fixed price for the Indigo crop.