Here are some sites that you may find helpful in preparing you to witness to Jehovah’s Witnesses:
Dawn Ellison’s website has interesting 10-20 minute videos analyzing teachings of the Watchtower.
4Jehovah is oriented to ex-Jehovah’s Witnesses and Christians who are trying to reach Jehovah’s Witnesses. There are a lot of good resources in its FAQ, Bible Answers, and Preaching sections. There are also a number of testimonies and talks that are free to listen to online.
Keith Walker has excellent articles on both Jehovah’s Witnesses and Mormons. I highly recommend his YouTube series of “Quick Questions for Jehovah’s Witnesses.”
David Reed’s website has a free download of his book, How to Rescue Your Loved One from the Watchtower, as well as several other free resources.
JWfacts has extensive documentation on the changes in Watchtower doctrine over the years, with photocopies of some of the key Watchtower documents. Also has information on current scandals involving the Watchtower.
First and Last is a UK site that is “a Christian ministry set up to reach Jehovah’s Witnesses with the Gospel of Jesus Christ.” Their blog has interesting articles, but fairly infrequent posts and not much commenting.
Wilbur Lingle has a PDF on responding to “What Does the Bible Really Teach?” that might be helpful to those who are studying the older book, as well as other articles on Jehovah’s Witnesses.
Christian Research Institute’s website has articles on Jehovah’s Witnesses from its Christian Research Journal, as well as information on many other cults and general apologetics.
Tim Barnett’s Stand to Reason is an excellent general witnessing/apologetics site. It has podcasts, audio, articles and a blog.