Hi Angela,
First one must learn what their "untruths" are, and then study up on the Biblical truths a bit. Basically they say that Jesus is not God, but a created angel. The Holy Spirit is a force, not a Person. Their "salvation" (which is not authentic) is by works,
if their false version of "Jehovah" thinks you've done enough for him and the Watchtower organization. One must put in so many hours of witnessing / handing out literature per week or month.
My take on their foundation is that their founder, Charles Taze Russell,
hated the doctrine of eternal punishment (he has those not approved by "Jehovah" annihilated), and the rest of the system built upon that.
If you just want to have them go from you, state and show from Scripture that the Lord Jesus is Jehovah the Son. For example, it is written of Jesus in Hebrews 1:10-12,
And, Thou, Lord, in the beginning hast laid the foundation of the earth; and the heavens are the works of thine hands: They shall perish; but thou remainest; and they all shall wax old as doth a garment; And as a vesture shalt thou fold them up, and they shall be changed: but thou art the same, and thy years shall not fail.
Yet it is said in Psalm 102 of God the LORD (i.e. Jehovah),
Of old hast thou laid the foundation of the earth: and the heavens are the work of thy hands. They shall perish, but thou shalt endure: yea, all of them shall wax old like a garment; as a vesture shalt thou change them, and they shall be changed: But thou art the same, and thy years shall have no end (vv 25-27).
So what is said of Jehovah God in the OT is revealed in the NT to be talking of Jesus Christ, who is Jehovah the Son, or God the Son. They may retort, "Well, yes, Jesus the angel of God was given to create the heavens and the earth, but this does not mean he is God." And in reply one should answer them, "But Psalm 102 says this specifically of Jehovah God, that
He created it all Himself, and we see in the NT that this creation by Jehovah was in fact Jehovah the Son."
One can also get copies on the internet of their old Watchtower and etc magazines showing where they made false prophecies about the end of the world, and how that the Watchtower organization was the prophet of Jehovah to give the truth to the world. Deuteronomy 18, in talking about false prophets says in v 22:
When a prophet speaketh in the name of the LORD, if the thing follow not, nor come to pass, that is the thing which the LORD hath not spoken, but the prophet hath spoken it presumptuously: thou shalt not be afraid of him.
And I will say to the JW, "You have proven yourself – that is, the Watchtower society of which you are a part – by your failed prophecies, to be a false prophet, and not to be 'feared', that is, not to be listened to as speaking for the true God."
One can also give them copies of these old Watchtower publications, but if you do they will not want to talk with you again. Once a JW is converted to Christ, he or she can be a powerhouse against the JW cult, and win many to the truth. So it does pay to have mercy on them, but one who deals with them must have a good knowledge of Scripture, and how to refute their teachings, and how to cause them to doubt by invalidating their sources of authority, as well as presenting the way of salvation in Christ in a manner understandable to them. It's not easy, as they are sharp, and often well-trained.