There are several different conflicts in Romeo and Juliet, but the main one, the one involving the protagonist, Romeo, can be described as character vs. circumstance, and this conflict is mainly portrayed through the plot and dialogue. By chance, Romeo was born into a family feuding with another family. It is true that Romeo's own personal choices led him to the ball, which led him to fall in love with Juliet as well as anger Tybalt. However, had the Montagues and Capulets not been warring with each other, it would have never been an issue for Romeo to appear at the ball, and Tybalt would never have felt insulted and angered. Therefore, as the plot shows, it was merely a matter of circumstance that Romeo entered a conflict with Tybalt, leading to both of their deaths. The moment when Tybalt challenges Romeo to a duel is the moment when the conflict is at its most intense moment and when the conflict is best seen.
We see the consequences of Tybalt having felt insulted by Romeo play out in Act 3, Scene 1. Here, dialogue Shakespeare uses to clearly portray the conflict can be seen in Romeo's lines when he declares to Tybalt:
I do protest I never injur'd thee,
But love thee better than thou cast devise
Till thou shalt know the reason of my love. (67-69)
These lines show just how much Romeo is caught up in circumstances. He has just fallen in love with and married Juliet, but her family, like her cousin Tybalt, hates him merely because he is a Montague. Romeo tries to walk away from the fight, but Tybalt felt so insulted by Romeo at the ball that he wants justice. Furthermore, Mercutio makes things even worse by taunting Tybalt, resulting in his own death as well as Tybalt's and, eventually, Romeo's. However, Tybalt had no genuine reason to feel insulted by Romeo or to hate him other than that their two families have been feuding for generations. Therefore, this fight with Tybalt perfectly portrays Romeo's conflict of character vs. circumstance.
Well Shakespeare comes right out and says what is going to happen in the prologue! He tells the audience through the chorus that two young people from feuding families will fall in love and then die.
Then, in the opening scene, Shakespeare shows the audience that conflict with a marketplace quarrel between the servants of the two families. The prince has to intervene and warn the heads of the two families that no more fighting will be tolerated.