Start with this book. Chapter 2 ("Inspiration: Choosing a Subject & Developing a Thesis") is filled with practical advice on identifying a subject you care about, narrowing your topic, and finding a promising thesis.
Scholarly Writing for Law Students: Seminar Papers, Law Review Notes, and Law Review Competition Papers, 5th ed.
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Your goal will be to write a comment, note, or paper that has something new to say, that is of manageable scope, and is interesting enough to merit as much as two years of your life.
The challenges are that the universe of potential topics is almost unlimited, that you are new enough to the law that you don't yet have a feel for many of the unexplored issues, and that you will, at the outset, be searching for something that can't be stated in discrete, descriptive terms.
This guide covers tips and techniques for identifying potential topics. Its main focus is on sources that will help you learn about what's new, unsettled, or controversial, and that don't require you to come up with specific search terms in order to delve into their content.
Make a list of intriguing topics as you go along. If any appeal to you, do some very quick searches of cases, statutes, books, and law reviews to assess the topic's potential. Is there enough raw material for you to analyze? Is the issue sufficiently complex to justify a paper, note or comment?
For research advice, see Law School Papers and Law Review Notes & Comments - Research Strategies.
If your initial research suggests that your topic has enough depth, do an initial quick preemption check to see if what you hope to say is sufficiently original to merit your time and effort.
Finally, if you are on the staff of a law review or journal at USF and do any source collection, see the Zief Library guide Source Collection for Law Reviews and Journals.