But soft, what light through yonder window breaks? It is the East, and Juliet is the Sun. (2.2.2-3)
He must have been with Rosaline last night
Why, then is my pump well flowered. (2.4.62)
Thou hast most kindly hit it. (2.4.57)
Come,come with me, and we will make short work, for by your leaves, you shall not stay alone. Till Holy Church incorporate two in one. (2.6.35-37)
Romeo, apparently having already forgotten about Rosaline, has snuck over the gate and into Juliet's garden, attempting to woo her with his words.
Benvolio and Mercutio are conspiring as to where Romeo was last night. They suspect he is still interested in Rosaline, and are teasing him for spending the night with her. They do not know he was with Juliet, or who she is.
Romeo and Juliet, despite knowing very little about each other, decide to get married. They have Friar Lawrence marry them. This cements their fate that is discussed in the prologue, this is the beginning of the doomed love that will destroy them both.