Here is a great way to witness easily and effectively to Jehovah's Witnesses. It gets them off their message track and makes them think:
But first make sure that when you answer the doorbell, you are polite and gentle and that you allow them to to speak their initial piece. Then ask them if you could ask a couple of questions. They will nearly always say yes. Using THEIR Bible (if they don't have a New World Translation with them, ask them to come back with one), open it to John 8:24. Here it will read "Therefore I said to YOU, YOU will die in YOUR sins. For if YOU do not believe that I am [he], YOU will die in YOUR sins.” Ask them if this means it is absolutely crucial to salvation to understand WHO Jesus really is, because if we get this wrong, Jesus says we will die in our sins. They will usually say yes it is important.
Now, Turn to Isaiah 44:6. Their Bible will read: "6 This is what Jehovah has said, the King of Israel and the Repurchaser of him, Jehovah of armies, ‘I am the first and I am the last, and besides me there is no God.'" Get them to agree that this verse in their Bible clearly says that Jehovah is the "first and the last" and there is no God beside Him.
Now go to Revelation 1:7-8. Their Bible will read: "7 Look! He is coming with the clouds, and every eye will see him, and those who pierced him; and all the tribes of the earth will beat themselves in grief because of him. Yes, Amen. 8 'I am the Al´pha and the O·me´ga,” says Jehovah God, 'the One who is and who was and who is coming, the Almighty.'” Get their agreement that Jehovah is indeed again in the New Testament saying that He is the "first and the last". Then ask them is it now Jehovah who "is coming in the clouds" and who was "pierced"? If they say yes, then point them to Matthew 24:30, 26:64; Mark 13:26, 14:62. Even in their Bible it clearly says in those verses that it is the "Son of Man" (Jesus) who is "coming in the clouds". So then, Jesus, the "Son of Man", is Jehovah? They will recoil at this and backtrack. But in all likelihood right from the beginning they will say verse 7 of Revelation 1 is speaking of Jesus, while verse 8 is speaking of Jehovah.
OK, then take them to Revelation 22:12-16. In their Bible it will say: "12 ‘Look! I am coming quickly, and the reward I give is with me, to render to each one as his work is. 13 I am the Al´pha and the O·me´ga, the first and the last, the beginning and the end. 14 Happy are those who wash their robes, that the authority [to go] to the trees of life may be theirs and that they may gain entrance into the city by its gates. 15 Outside are the dogs and those who practice spiritism and the fornicators and the murderers and the idolaters and everyone liking and carrying on a lie.’ 16 ‘I, Jesus, sent my angel to bear witness to YOU people of these things for the congregations. I am the root and the offspring of David, and the bright morning star.’” Point out to them that JESUS is saying He is both "the Alpha and Omega" and "the First and Last". They will likely try to explain this passage away by saying that it is Jehovah speaking in verse 13 and Jesus in the rest of the verses. Do not argue with them. Say OK, but they have to agree that whoever it is that is saying He is both the Alpha and the Omega and the First and the Last is still Jehovah.
Now turn in their New World Translation to Revelation 1:12-16. You will read: "12 'And I [my note: this is John speaking] turned to see the voice that was speaking with me, and, having turned, I saw seven golden lampstands, 13 and in the midst of the lampstands someone like a son of man, clothed with a garment that reached down to the feet, and girded at the breasts with a golden girdle. 14 Moreover, his head and his hair were white as white wool, as snow, and his eyes as a fiery flame; 15 and his feet were like fine copper when glowing in a furnace; and his voice was as the sound of many waters. 16 And he had in his right hand seven stars, and out of his mouth a sharp, long two-edged sword was protruding, and his countenance was as the sun when it shines in its power.'" Ask them who this is that John saw. They HAVE to say Jesus. (If there is any doubt, the context of the entire first three chapters of Revelation makes it clear it is Jesus speaking.)
Now read the next verse, verse 17: "And when I saw him, I fell as dead at his feet.And he laid his right hand upon me and said: 'Do not be fearful. I am the First and the Last.'" Now point out to them that clearly Jesus here is claiming to be the First and the Last. But remind them that in Isaiah we read that Jehovah said He alone is the first and the last ... and then repeats that claim all through the verses we have read in Revelation. So how can Jesus be the first and the last? There can't be two firsts and lasts. Only one can be "first"; and only one can be "last".
Now they will be at a loss here ... but will still somehow be trying to say Jehovah IS the "first and the last". Now ask them one more thing. Read them verse 18, where the One who is first and last continues to speak and says: "'and the living one; and I became dead, but, look! I am living forever and ever, and I have the keys of death and of Ha´des.'" Now ask them "When did Jehovah die?" It clearly says here that the one claiming to be the first and the last died and is now alive again. So when did Jehovah die?
Trust me, this should be enough to totally shake their theology regarding the person of Jesus.
Now remind them of that very first passage they explained to you (John 8:24) where the one (whom they clearly told you was Jesus) said that those who do not believe who He really is, will die in their sins.
You now have the opportunity (presuming they haven't run off by now) to explain John 1:1 and that the contradiction you placed before them can only be removed by recognizing that Jesus and Jehovah are one and the same. You can then offer to do a study with them PROVING that the two are one ... and that the ONE God is eternally self-existent in THREE persons. And if they are really seeking the truth and want to ensure that they do not die in their sins because they are not recognizing who Jesus really is, then you will be willing to share with them the true gospel of Jesus Christ.
Whether they agree to either a) continue, or b) just leave, make SURE you pray with them before they go... asking in the Name of Jesus that the One True Almighty God open their eyes to the truth of the gospel of His salvation and the real identity of Jesus Christ.
God bless you, and I pray you can make use of the above very simple (Revelation 1:7-8; Revelation 22:12-16; Revelation 1:12-16, 17, 18) method above.