Assuming that the case you are expected to write an essay on is Keeler V. Superior Court, 2 Cal.3d 619, 87 Cal. Rptr. 481, 470 P.2d 617 (1970), you will need to decide what main point you want to make about this case and decide how you want to support that point, just as you would for any other essay. Similarly, you will need an introduction, body paragraphs to support your idea, and a conclusion.
When you are writing an essay about a court case, you must take particular care to inform the reader about the case and not assume the reader is familiar with it. Explain to the reader who the parties in the case are, give the facts of the case, and let the reader know what the holding of the court is. In this case, the court held that a fetus is not a "person," which means that when someone causes harm to a fetus that results in a fetus's death, that person cannot be charged with homicide. You will want to explain all of this in the first body paragraph after your introduction.
Perhaps your main idea will be that you agree with the court's decision or that you disagree with the court's decision. Or you might want to focus on the aspect of the case that is called legislative interpretation, how a court interprets the language of a statute. You could write an essay on what should be done to prosecute someone who harms a fetus, if this statute doesn't work to accomplish that. Those are some possible main points you can pursue in the essay.
Once you have written an introduction and explained everything a reader needs to understand about the case, the rest of your body paragraphs should support your main idea. Was the court correct? Why or why not? Explain your point of view on this issue. Have several ways to support your position, and use one body paragraph for each way you support that position.
Finally, in a concluding paragraph, wrap it all up in one neat package, reminding the reader what your main point is and how you supported it. Good luck!