I think it is important to understand the characteristics of a tragic hero to write your essay.
A tragic hero will often be noble, not necessarily in social status, but in terms of character. Romeo fulfills this as he puts the feud of the families behind him. He asks Benvolio in the first act about the "fray" that he missed. He tries to give Tybalt reason in Act 3 for loving, rather than calling him a villain. Sometimes this nobility turns out to be a flaw as we saw Tybalt take advantage of it in Act 3.
A tragic hero will also experience consequences based on his own errors. Romeo ends up returning to Juliet's tomb and killing himself because he was not patient enough to wait for the Friar's messages. It was certainly a problem that the message could not get through, but Romeo did not need to rush into killing himself. This impulsive behavior was a flaw that plagued Romeo throughout our experience watching him on stage.
A tragic hero often knows that his fate is sealed by doom and downfall. This flaw is a problem because it is a self-fulfilling prophesy. Both Romeo and Juliet foreshadow their own and each other's deaths throughout the text. The reason this is a problem is because it is like because they say it, they begin to believe it. Juliet mentions this when she says her goodbye to Romeo in Act IV, Romeo mentions it in his dream in Act V, scene i. They both seem to reference death in Act I scene v and Act II scene ii.