To translate this quote into Modern English, Juliet is saying, "My only love springs from my only hate! I saw him too early when I didn't know him, and now I realize who he is too late! This is a horrible beginning to love that I must love a hated enemy," as taken from the Romeo and Juliet Parallel Text, Second Edition from Perfection Learning.
Juliet is saying that she has never loved before, so Romeo is her only love, and the only hate she has ever known has been the hate towards the Montagues that she has been raised by her family to feel which is the family Romeo comes from. She saw Romeo before she identified him with the hated family and fell in love with him before she knew to stop herself. Juliet feels that it is awful that the first love she has ever felt ends up for a member of the rival family, the Montagues.