Without knowing your purpose, it is difficult to direct you on your thesis statement.  However, here are a few suggestions:

If you are writing about the causes, you could ask a question in the introduction such as How did a wig and a rock effect a tragic event in American history?  (As you know, the Massacre began after a wigmaker's apprentice, Edward Gerrish, called out to a British officer, Captain Lieutenant John Goldfinch, that he had not paid his master's bill; and, another fatal incident occurred a few days before the Boston Massacre when Christopher Seiden was shot on February 22, 1770, when a fight began between an angry mob and British royalists who threw rocks at the shop of a Loyalist merchant.)  Then, formulate a thesis of three important points (if you are writing the 5 paragraph essay) on the idea of odd occurrences as the causes of the Boston Massacre.

Or, if you are writing a descriptive essay, you could write a thesis about facts that are not commonly known about the Massacre such as the unusual outcome of the trial of two British regulars whose defense centered upon 'benefit of the clergy'; they were proven to be able to read the Bible and their charge was changed from murder to manslaughter.  However, they were stamped with an "M" to prevent further such cases.  Review the site below for more unusual facts.

Certainly, this massacre is one of those historical events about which Leo Tolstoy wrote in the latter part of his great book, "War and Peace."  Tolstoy reflects upon how events occur not because of anything, but simply as a human reaction to sometimes unrelated forces outside or some unified force within themselves.  Perhaps, it is the "tide in the affairs of men" of which Brutus speaks in Shakespeare's "Julius Caesar" (Iv,iii,217) that causes such historical events.

Well, start with what a thesis statement is--and is not. A thesis states an idea (not a fact). It states an idea that you believe you can explain and support with evidence that you gather through research. It is the idea that your paper will explain and support through several main points of discussion. By explaining and supporting each main point, you then will have explained and supported your overall thesis. Think of it as a math formula:

Point 1 + Point 2 + Point 3 = Thesis Statement

So, you should begin by asking yourself what you believe to be true about the Boston Massacre, in terms of ideas (not facts). Work from some questions. Did the Boston Massacre push the colonies into war? Is the importance of the Boston Massacre overstated? Was the Boston Massacre one of the most important events in the American Revolution? Could the Boston Massacre have been avoided? Are the actual events of the Boston Massacre generally misunderstood?

There are many, many questions you could formulate, and your answers (based upon what you have learned through study and research) would become thesis statements. For example, did the Boston Massacre push the colonies into war? If you believe this is true, then here's your thesis:

The Boston Massacre pushed the American colonies into war with England.

If you don't believe this is true, then your thesis might be this:

Although the Boston Massacre is historically important in the American Revolution, other forces played a larger role in pushing the colonies into war with England.

Just remember that your thesis is the answer to a question, an answer that you can explain and support with facts and evidence. I've included some links below to take you to some excellent enotes sources that will help you with your research. Good luck!