You will need to think through the character of Brutus, and in particular his motives for wanting to depose Caesar. There are many different ways of viewing his character, but one valid approach is to explore the way that Brutus is actually tremendously naive. He has lofty motives for deposing Caesar, but his innocence and naivety makes him easy prey for the flattering tongue of Cassius. He is used by the plotters to gain credibility for their less-than-noble actions yet he remains unaware of the way that he is being used. He believes all the while that he himself is trying to do what is right for Rome, but we as the audience cannot help seeing another more questionable agenda at work. Brutus, like Caesar before him, is prone to the same power-hungry tendencies of anyone given such unlimited control and might. Thus a good thesis statement might be:
Brutus is the tragic hero of this play because of his naivety and innocence, both in regard to other people and himself.
This would allow you to explore the differing approaches I have outlined above and will make for an interesting essay. Good luck!