Explain how Romeo finds out about the Capulet ball.

In Act 1, Scene 2, Lord Capulet says to his servant,

“Go, sirrah, trudge aboutThrough fair Verona, find those persons outWhose names are written there, and to them say,My house and welcome at their pleasure stay.”

Here, Capulet tells his servant to invite the people on his list to the ball that will be held at Capulet’s house. As the servant walks through the townsquare, uncertain what to do because he is illiterate, he runs into Benvolio and Romeo. He asks them to help him read the list so he knows who to invite to the Capulet Ball. Benvolio and Romeo help the servant, and in doing so, find out the details about the party. Benvolio then tries to convince Romeo to attend with him and Mercutio as a way for Romeo to get over the girl who broke his heart, Rosaline. Benvolio tells him that it would be good for Romeo to go to the party and see that there are so many other girls who are just as beautiful and wonderful as Rosaline, but Romeo continues to refuse to go because all he can do is think about how heartbroken he is. Romeo finally agrees to go to the party but only because he feels there is a small chance he could see Rosaline there.

Romeo finds out about the Capulet ball from his friend Benvolio. Benvolio has been approached by Romeo's father, who is worried about why Romeo is mooning around all the time. Benvolio explains it is because of his unrequited love for Rosaline. Benvolio also says he will intervene and try to help Romeo.
Benvolio invites Romeo to the Capulet ball in an attempt to interest him in girls other than Rosaline. He tells Romeo that there are many beautiful girls in Verona and that he should try to meet them, rather than moping over Rosaline. Romeo insists there can never be any other girl in the world for him but Rosaline. He nevertheless goes to the Capulet ball with Benvolio and another good friend, Mercutio.