It might be best to plan for a slow exit strategy instead of slamming the door, since this might cause some negative reaction from your family.
However, if you do want an immediate end, then i suggest taking the approach of telling your family that according to what you see, the Watchtower society does not have Jehovah's blessing and is not following his direction.
In my experience this disarms JWs, since the thought of "leaving Jehovah" does not fit well within that concept. Of course this means, you will need to be able to support your position by picking 2 or 3 incriminatory points and be able to defend them with evidence.
I did this myself initially and my family, didn't exactly know what to say, since my point was that I wanted to do what Jokehova wanted, and thus I could not follow the Watchtower.