Okay, being a teacher, I need to begin with a plea for you to PLEASE read the actual young adult novel series that the show is based upon! I think you'll find the series is even better than the show!!! (See the link below!)
As for the TV episode, you can try looking at the ABC Family website. I checked there as well, and they have plenty of full episodes right online for you to watch! Often TV websites post the full episode directly after it airs for your convenience.
If you have a subscription to a streaming service such as Netflix or Hulu, you can also look for the episode there. Or you can try your local video rental store or library!