In Act I, Scene 2, Caesar walks through the streets with other Senators during the feast of Lupercal. A soothsayer tells Caesar to "Beware the ides of March" (I;ii;in 18). Meanwhile Cassius starts talking with Brutus and Casca of a plot to kill Caesar. Brutus says he needs time to think.
Act IV: Sc iii
In Act II, Scene 1, Cassius and the other conspirators visit Brutus at his home. Brutus welcomes them all and joins the conspiracy by shaking their hands, saying "Give me your hands all over, one by one" (II;i;in 112). They then discuss their plan for the assassination. Brutus ultimately takes control of the group.
Act V: Sc v
In Act III, Scene 1, Caesar is at the capitol with all the conspirators. They all gather around Caesar. Casca says "Speak hands for me!" (III;i;in 78) giving the call for the attack. Then they all stab Caesar, killing him.
In Act IV, Scene 3, Brutus is trying to sleep before going to march the next day. He is visited by the ghost of Caesar. Caesar's ghost tells him "to tell thee thou shalt see thee at Philippi" (IV;iii;in 283). THis makes Brutus decide to march to Philippi immediately.
In Act V, Scene 5, Brutus' army has basically lost the battle. Brutus gets a friend to help him kill himself. Octavius and Antony arrive and find Brutus dead. Antony says "This was the noblest Roman of them all" (V;v;in 68) about Brutus.
The theme of Julius Caesar is to not believe everything you are told. In Act II, Scene 2, Caesar has decided to stay home from the Capitol because of his wife's dream. Decius comes and tells him "This dream is all amiss interpreted; It was a vision fair and fortunate" (II;ii;in 83-84). Caesar believes him and goes to the capitol. If he had been less gullible, he wouldn't have been killed.