Balthasar had told Romeo that Juliet died, which is not the case. So Romeo decided to buy poison, so that he could kill himself, so that he would be with Juliet. Romeo went to the Apothecary to get the poison. Having this poison however, has many consequences, and the Apothecary was very hesitant in giving it to him. But since Romeo offered quite a bit of money, he did give it to Romeo.
Paris was heading to Juliets' tomb, to lay flowers in her memory. He is also there to say his final goodbyes. He did in fact truly care about her and is now grieving her death.
Death, that hath suck'd the honey of thy breath, hath had no power yet upon thy beauty: Thou art not conquer'd, beauty's ensign yet is crimson in thy lips and in thy cheeks, and death's pale flag is not advanced there.
Romeo and Balthasar are heading towards Juliets' tombs as well. Romeo then tells Balthasar to give his father a letter that he wrote. He also said that he Balthasar should just leave him alone no matter what might happen.
What’s here closed in my true love’s hand? Poison, I see, hath been his timeless end. O churl! Drunk all, and left no friendly drop To help me after? I will kiss thy lips. Haply some poison yet doth hang on them.
As Paris walks to Juliet. he sees Romeo. Paris assumes that Romeo is there to steal jewelry, which is not the case. Romeo does repeatedly tell him to leave him alone, as he is going to kill himself. But Paris did not leave, and Romeo ended up killing Paris. He then laid Paris in a tomb.
After Romeo and Paris fought he went over to Juliet, and then said a speech, before he drank the poison. He had said that she does not look dead, and she still had the colour in her cheeks, and that death did her well. Romeo then drank the poison, and he died instantly.
A couple minutes after Romeo died Friar Lawrence had come to be there when Juliet woke up. He then saw Romeo had died. Juliet then woke up, and Friar Lawrence tried to get her out of there, but she then saw Romeo, she then decided to kill herself as Romeo did. She then stabbed herself, and had died.