This depends to some extent on what you want your essay to be about. You could go in a number of directions. For example, you could discuss any or all of the following:
- The extent of damage done and the plight of the survivors. You could focus your essay on a description of how bad the disaster was and on what was done or not done to help the victims.
- The actions or inactions of the Indian government that allowed the disaster to happen. You could research the extent to which the government did or did not try to ensure that the Union Carbide plant would be safe.
- The degree to which Union Carbide itself was responsible for the disaster. You could look at the evidence that shows that the company was aware that the plant was dangerous. You can try to account for why the firm did not do more to ensure that the plant was safe.
These are some areas that you might address. Please follow the link for good details on the circumstances surrounding the disaster.