You need to work out first what is most important to them. How much faith do they have in the Watchtower, and how much in the Bible. What makes them think the Bible is inspired, and what makes them think the Governing Body are directed by God? What are some of their doubts? Then you will know what topics to research and discuss.
For instance, if they have questions about the flood, or evolution, you can then discuss how Genesis cannot be taken literally. If they have questions about the history of the Watchtower religion, or talk about there always being JW's, you can go down that path. Maybe they wonder why 100 years have passed since 1914 and nothing has happened, then it could be a doctrinal discussion about 607.
I personally started to doubt the topic of universal sovereignty, and also whether God answered prayers. Yet did not question the Governing Body. It took a range of information to finally get through.