Start a very calm and respectful conversation with your Dad along the lines of freedom of religion. Get him to tell you/agree that baptism is a choice that has to be made freely without any form of coercion or pressure. Maybe even mention the bad example of some people - of other religions - forcing their children into their religion against their will. Get the May 2009 Awake (hope I have the right issue date) that says persons should not be forced to join a religion and that persons should not be made to choose between their beliefs and their family.
After you have done all that very calmly and respectfully tell him that you have given the subject of baptism a lot of thought and come to the decision that you do not wish to get baptized, and ask him to please respect and accept your decision since he already stated that you're old enough to make a decision and he has already indicated (from the foregoing conversation) that he accepts you have the right to freedom of religion and should not face any form of coercion to be baptized or penalties for refusing to do so, because parents that truly respect their children's right to choose would not engage in such actions, and you have confidence that he's not such a parent - based on the foregoing conversation.
If he asks you why you don't want to get baptized, just very calmly tell him that you do not wish to discuss it as he may perceive what you have to say as attacking his religion and he will get upset. Tell him you respect his right to be a JW and you're not out to discourage him or destroy his faith so you rather keep your reasons to yourself so as not to upset him in any way. You only want him to respect your decision and your freedom of choice to not become a JW, just as you respect his right to be JW.