where do you find serana in skyrim?
Serana can be found in Dimhollow Cavern. You can tell Serana from the scroll that she has on her back. Upon introduction, she can easily identify you as a vampire or not and will request that you take her home.
You must begin the quest "Awakening" to meet up with Serana. You will find her in Dimhollow Crypt. http://elderscrolls.wikia.com/wiki/Awakening