Was Brutus truly the main character in the play? I get the feeling that he was.
When you "get a feeling," it's good to explore it by returning to the text. I am going to help steer you to defend your "feeling" (let's call it an interpretation) by asking you some guiding questions so that you can build your own analysis.
First, let's ask, what makes a character "main" -- a protagonist? Ask these questions: Does Brutus face a key conflict in the story? Does he take important actions that have big consequences?
Note in act 1 how Cassius approaches Brutus with a very important observation -- that he hates Caesar and wonders why a man who is clearly not a "god" should be such a Colossus." This is in response to Brutus expressing a fear that the people want Caesar to be their king. So, there is the conflict for Brutus, and I would advise you to return to that first conversation between Brutus and Cassius to see the two fears Brutus expresses. I've shared the first one with you; can you find the other? These two fears create a struggle, or conflict, for Brutus.
How does Brutus act upon those fears? What choices does he make? Look at acts two and three for the answers.
The other person who posted a response here says that Caesar is the main character because the title of the play is "Julius Caesar." That's a strong piece of evidence, but you might have got your feeling that Brutus is more important because Brutus is still alive before the end (but not at the very end) of act 5, while Caesar died in act 3.
In literary analysis, we can argue many different interpretations, but the question you face as a student is, How can I best defend my answer? So, after you finish defending the idea that there is a big conflict Brutus faces (prove that), and then you prove that he takes important actions that render significant consequences (again, find evidence to prove), what's next is to look at other pieces of evidence:
-- what others say about Brutus. A character's reputation is important, and what others say and do regarding another character can tell you just how important s/he is (for example, how good or evil on a grand scale. The more virtuous or villainous, the more the character is "larger than life" and thus an important or main character.) What do the other conspirators say about Brutus when they are plotting Caesar's murder in both acts 1 and 2? (Look at Cassius's words to Casca about Brutus's reputation, and look at what Caius Ligarius says to Brutus when he comes sick with illness to Brutus's house.) What does Antony say about Brutus in the final lines of the play?
-- Also examine this question: is it possible that Shakespeare's play might have two main characters, or is there only room for one? Critics often point to the complexity of Shakespeare's characterizations, so that lets us know there may be room for two starring figures.
If you visit the Julius Caesar discussion boards, I respond there with a note about what Elizabethans already knew and believed about Julius Caesar as a heroic historical figure. Shakespeare could have made Brutus and Cassius very minor figures in the story and spent more time on Caesar, but instead he made all three men quite interesting. Hmmm....
Good luck answering these questions. There's a lot of rich material to explore in this play, so enjoy!