Romeo says the line to Tybalt, "Either thour or I , or both, must go with him," immediately after Romeo's best friend, Mercutio, has been slain by Tybalt who has just returned to the scene of the crime. In order to better understand Romeo's line, we must first understand it in context.
Immediately after having stabbed Mercutio, Tybalt turns and runs from the scene. After Benvolio announces Mercutio's death, Romeo stands dumbfounded, knowing that Mercutio's death is about to bring a black fate for Romeo as well. When Tybalt returns to the scene, Romeo knows he will avenge himself on Tybalt for his friend's death, as we see when Romeo proclaims, "Away to heaven respective lenity, / And fire-eyed fury be my conduct now! (III.i.124-125). In other words, Romeo is declaring that he is furious and about to act on his fury. He then tells Tybalt that "Merctio's soul" is "above [their] heads," meaning in heaven, and waiting for Tybalt's soul to keep him company (127-129). In other words, wanting revenge, Romeo is challenging Tybalt. His challenge continues with the line, "Either thou or I, or both, must go with him" (130).
In the line, "Either thou or I, or both, must go with him," Romeo is saying that either Tybalt must go with Mercutio, meaning must die as well as Merutio, or he, himself must die with Mercutio, meaning that Tybalt will kill Romeo as well. Finally, Romeo also says that the other option is that they both must die with Mercutio as they both may kill each other. In other words, Romeo is warning Tybalt that more deaths are about to happen as a result of Romeo's vengeful dual. Either Romeo will die, Tybalt will die, of they will both die.
I'm guessing it's something like, "Either you, me or both of us must go with him."