Most plots follow a certain structure containing five elements: (1) exposition in which the characters, situation, and conflict/conflicts are introduced, (2) rising action, which are the events that lead up the most dramatic point in the story, (3) climax, which is the story's turning point, the most dramatic moment in the story, the point at which the resolution is in sight, (4) the falling action, which are all the events that lead to the final resolution, and (5) the resolution, which is the final ending of the story, the moment that all lose ends are tied. When looking at plot development, you are looking to see how the story is progressing with respect to these five elements.
The first three scenes act as the exposition because in these scenes most of the principle characters are introduced. More importantly we are introduced to several different conflicts. The first conflict is man vs. man, and not only are we introduced to this conflict through the servants' fight, we are especially introduced to it during Prince Escalus's first speech. In this speech he warns, "If ever you disturb our streets again, / Your lives shall pay the forfeit of the peace" (I.i.92-93). This threat is central to the plot development because it causes a lot of the strife that happens later, particularly Romeo's banishment and the deaths that result from his banishment.
A second important conflict we are introduced to in this act can be considered man vs. man or man vs. circumstance. The conflict pertains to the fact that Paris is asking for Juliet's hand in marriage. While it doesn't seem like such an important conflict at first due to the fact that Lord Capulet refuses to consent to the marriage just yet, later we see it is significantly important and even a contributing factor to both Romeo's and Juliet's deaths. Hence we see that the moment Paris asks for Juliet's hand in Scene 2 of Act 1 is significant to the plot development because it lays out another conflict. This is a conflict between Juliet and both her father and Paris, making it a man vs. man conflict. But it also is a conflict between Juliet and her circumstances or fate, making it a man vs. circumstances conflict.