It isn't clear how far you've gotten in the play, but it sounds like Romeo hasn't yet made it to Mantua. (He will eventually be exiled and make a temporary home there.) You'll have to decide whether to strike a more modern, casual tone ("You absolutely won't believe who talked to me at the Capulet bash!") or a more formal and Shakespearean tone ("It is with great joy that I write to you regarding my recent romantic interests"). The prompt also doesn't clearly define which character you are supposed to be, so I think Romeo would make a great voice here.
Here are some plot points before his exile that he might want to share with his cousins:
- Romeo has finally gotten over Rosaline. When the play begins, everyone is concerned about his morose attitude; his friends encourage him to move on, but Romeo is really pining for the woman who has rejected him
- He attends a party hosted by the Capulets and finds the girl of his dreams: Juliet. Juliet flirts with him, and they kiss just moments after meeting
- Before leaving, Romeo finds out that Juliet is the daughter of Lord Capulet, whom the Montagues hate. He can't believe his bad luck in falling in love with a Capulet
- Juliet is not yet fourteen when Romeo falls in love with her. He doesn't seem to be concerned with her young age, and he might mention something about it to his cousins and explain why he's not concerned about it
- Romeo is so captivated by Juliet that he sneaks over to her house after the party. Standing in the orchard, he sees her come outside onto her balcony. She begins talking about how much she adores Romeo and swears that she would give up the Capulet name to be with him. This is all the motivation Romeo needs, so he comes out of hiding and tries to tell her that he has fallen for her, too. Juliet is worried that they are moving too fast, but Romeo eases her concerns. They begin talking about marriage, and Romeo leaves, planning to talk to the friar
- The friar is shocked that Romeo wants to marry Juliet before his tears over Rosaline have time to dry. Romeo defends his choice, saying that the friar (and all of his friends) have been pushing him to forget about Rosaline, anyway. Romeo finally convinces the friar to perform the wedding ceremony
- Romeo leaves to hang out with his friends, who taunt him about where he had been the night before. Joking around with Mercutio, Romeo's demeanor has clearly changed, but he refuses to tell them about Juliet. When Juliet's nurse arrives, Romeo tells her that he has genuinely fallen in love with Juliet. He asks the nurse to have Juliet meet him that afternoon for the wedding. This would be a good point for personal reflection. Do you think Romeo had any doubts about Juliet showing up? Is he convinced at this point that she is truly willing to give up her name?
- The young lovers are married in secret.
After this, Romeo engages in conflict with Tybalt, which leads to his banishment, so I wouldn't think you could go beyond this plot point. Be sure to weave Romeo's voice into the plot details. Be sure to convey his excitement, confusion, fears, and distress. Does he fully trust Juliet? How does he think all this will turn out? Does he believe that Lord Capulet will accept him as his son-in-law after the wedding?
In closing, Romeo might ask his cousins if they can give him a place to stay if things with Lord Capulet don't go well.