Your introduction should do several things. First, it should grab your reader's attention. Compelling quotes are a good way to do this right from the start. Make sure to footnote any quotes, as well as all other information that is not your own original thought or conclusion. Second, the introduction should clearly explain your purpose in writing about your topic, the issue(s) you will address, and why these issues are of interest. Finally, your introduction should provide the reader with a clear roadmap of your paper.
The background section should go in to detail about the background of the problem / area of the law you have chosen to research and write about. What are the issues and where and how did they arise? What impacts, both intended and unintended, do these issues have on society? What are the historical underpinnings of the issues? Why does it matter? This list is not exhaustive, but rather is intended to get you thinking in the right direction.
Remember that you will be citing to your authority using footnotes in Bluebook citation format. Expect to have quite a few footnotes in each paragraph that you write, as legal writing is generally much more citation-intensive than other types of writing.