Welcome! It depends on how old you at and if you are financially dependent on your mother. If you are an adult and on your own you can be a bit more direct obviously.
If it were me I would say that although you appreciate all that she has done to raise you right, you simply do not share her belief in this religion. Tell her that she raised you to be honest and you cannot live a lie. You have prayed about it and looked for answers in the publications, but there is just no way you can close your eyes to the facts anymore. You should assure her that you still love her and do not want to live a life of sin, but that you will not ever be a Jehovah's Witness.
I am sure this will be very difficult, but it's much better to be open and honest about your beliefs and not try to hide it from her. She will be disappointed, but she will just have to adjust and accept that you are an individual and have to make your own way inlige, just as all parents have to accept the decision made by their adult children.