Just say that you happened to come across some things which made you rather confused about the Watchtower organization, such as their history of __________, and their apparent contradictions regarding __________, etc., etc. Say that in consideration of those things which you have fairly recently come to know of, you will therefore need more time to analyze the facts and process your thoughts on these matters. That way, your execution – I mean, baptism – can be put on hold, and you will still retain a good measure of immunity from being shunned.
Remember, they can’t kick you out of something from which you technically never joined in the first place (that is, according to their own rules).