Identify a character who serves as a foil in Act One, Scene One of Romeo and Juliet, and explain how Shakespeare creates the foil.

First, let's identify what a "foil" is within this context. In literature, a foil is a character whose qualities directly contrast the qualities possessed by another character; in doing so, the foil's qualities draw attention to and highlight the qualities of the other character. Foils are generally secondary in nature to a protagonist. 
In Act One, Scene One of Romeo and Juliet, two servants of the Capulets—Sampson and Gregory—begin a fight with two servants of the Montagues when Sampson bites his thumb at them. This brawl is quickly joined by various members and friends of the rivaling families. While Benvolio (a friend of the Montagues) attempts to call for a stop to the fighting, Tybalt engages him in a fight of their own while declaring his hatred for peace. Montague and Capulet themselves attempt to go after one another, with only their wives preventing this. The fight is only formally broken up when Prince Escalus angrily commands the two parties to lay down their weapons, and declares that anyone who disturbs the peace in such a manner again will be sentenced to death. After the crowds disperse, Benvolio goes to comfort Romeo, who is incredibly depressed over his unrequited love for Rosaline, a young woman who has sworn to live a chaste life. 
We can see from this scene the two characters which are in clear opposition to one another: Benvolio and Tybalt. We can argue that Tybalt is a foil to Benvolio, or that Benvolio is a foil to Tybalt—depending upon which character we identify as more "primary" in nature. I would personally argue that Tybalt is more of a primary character in that he largely operates as the antagonist throughout the play. 
Thus, Benvolio is the foil to Tybalt. While Tybalt is fiery, impulsive, explosively violent, and arrogant, Benvolio is calm, even-tempered, logical, and always in favor of the common good. We see this in Benvolio's initial response to the fight, when he comments:

Part fools!
Put up your swords. You know not what you do.

Meanwhile, Tybalt responds to this call for peace with a swift negation:

What, drawn, and talk of peace? I hate the word,
As I hate hell, all Montagues, and thee.
Have at thee, coward!

Tybalt clearly has no interest in resolving conflict, while Benvolio does everything in his power to prevent the fight from progressing. Benvolio's ability to remain reasonable in this situation—and the fact that he really only physically engages with Tybalt as a matter of self-defense—makes Tybalt's violent flair all the more evident.