I'm not sure how specific you need to be here, but it's quite obvious that Mark Antony is the best example of an effective orator in Shakespeare's Julius Caesar.
Immediately after Caesar's murder, Brutus is able to persuade the angry Romans that Caesar was ambitious and that his death was justified. However, Mark Antony, to whom Brutus has given permission to speak, is able to convince the Romans that Caesar was wrongly murdered.
If you're looking to discuss both Brutus and Antony, you'll want to create a thesis statement that discusses ways in which they appeal to the Romans' values (or beliefs, etc.) in Act 3. If you want to be more specific, you might consider something like this: In Act 3, Mark Antony uses verbal irony (focus on his use of the word "honorable") and persuasion (focus on the tactis he uses to convince the Romans that Caesar's murder was unnecesary--he shows them the stab wounds, reads them Caesar's will, etc.) to influence the Romans' thoughts about the conspirators' murder of Caesar.
Hope this helps. Good luck with your essay!