The first step to writing a good paragraph is coming up with a topic sentence. Every other sentence in the paragraph should refer to that topic; if it does not, it should be omitted from the paragraph. In this particular case, an appropriate topic sentence might be something like, "In Act One, Scenes One through Three of Romeo and Juliet, plot developments include the exposition and some elements of the rising action of the play." Of course, that is not the only option.
In writing this paragraph, I would mention the information that is given in the exposition, such as the fact that the Montague and Capulet families are feuding (violence three times), another outbreak of violence will result in executions, etc. Important examples of rising action, which are leading to the climax of the play, should also be addressed. Some examples of the rising action in these scenes include the fight between Benvolio and Tybalt, Paris's desire to marry Juliet and Juliet's mother's agreement to the idea, and Benvolio and Romeo's plan to "crash" the Capulet party.