For Act I of Romeo and Juliet, in what poetic form is the prologue expressed?the first prologue

The poetic form of the English sonnet, as described nicely by these other posts, was used so often in William Shakespeare's plays and works that it has become known as the Shakespearean sonnet. This is the most widely known type of sonnet, although the sonnet was originally invented in Italy, and the English sonnet was introduced by the poet Thomas Wyatt in the 16th century. Yet, Wyatt is not known as the person most connected with the English sonnet. That honor goes to the figure of Shakespeare. It is also known as the Elizabethan sonnet, because it's popularity exploded during the era of queen Elizabeth, known as the Elizabethan Era. 

The Shakespearean sonnet is set in iambic pentameter as the metrical foot, but it is not metrically inflexible. There are some small variations in the meter. The overall rhythm of the meter helps to set the scene by sounding purposeful.

Sonnets were (and are) generally popular because of this meter and their well-known ubiquity. They are traditional forms used to express ideas and feelings on love, which happens to be perfect for the themes explored in Shakespeare's Romeo and Juliet. Remember also that Romeo and Juliet, as well as other Shakespearean works, were not simply literary works meant to be respected and admired for linguistic value. They were PLAYS. Which means they were seen on stage, for entertainment value and purpose. The audiences of these plays would respond to a sonnet well because rhymes and rhythm are accessible. 

Two households, both alike in dignity,
In fair Verona, where we lay our scene,
From ancient grudge break to new mutiny,
Where civil blood makes civil hands unclean.
From forth the fatal loins of these two foes
A pair of star-cross'd lovers take their life;
Whose misadventured piteous overthrows
Do with their death bury their parents' strife.
The fearful passage of their death-mark'd love,
And the continuance of their parents' rage,
Which, but their children's end, nought could remove,
Is now the two hours' traffic of our stage;
The which if you with patient ears attend,
What here shall miss, our toil shall strive to mend.