Evangelist,
When dealing with JWs, I recommend you initially use their Bible, because they don't trust anything other than their translation. The good news is that even their own Bible supports the idea of the deity of Christ, though most JWs and most Christians are not aware of it. Therefore, first ensure that the JW you're speaking with agrees that their Bible, the New World Translation (NWT) is authoritative and trustworthy, then present them with the following verses as written in the NWT.
Jn 5:17-18. Jesus calls God His Father, and the Jewish religious leaders prepare to stone Him. Why? Because as the verse says, they realized that Jesus, by calling God His Father was making Himself the equal of God. They wanted to stone Him because they considered what He was doing blasphemous.
Jn 14:7. Jesus tells His disciples that to see Him is to have seen the Father. Why? Because in their essence, Father and Son (and Holy Spirit) are One God.
Isa 9:6. This is one of the most famous verses in all of Scripture and it clearly says that the baby born in Bethlehem will be called, “Mighty God.” Note the use of the capital G, a clear indication by JW standards of a reference to deity.
Matt 1:23. The baby born to the virgin Mary was to be called, Immanuel, meaning, "With us God (or God with us)." Jesus was God with us, having stepped briefly into our space-time continuum.
Jn 20:27-29. Here, the disciple, Thomas, the one who doubted, called Jesus his Lord and God. If he had been mistaken, Jesus could have and would have corrected him, but instead, Jesus tells him that those of us who believe without seeing Him will be happy. And what will we believe that will make us so happy? The same thing Thomas realized, that Jesus is Lord and God.
Rev 1:8; 21:5-7; 22:13. Each of these makes reference to Jehovah as Alpha and Omega, First and Last, Beginning and End, and most good JWs will agree that Jehovah God is the One Who is being referred to. However, in Rev 1:17, the speaker refers to Himself as the "First and Last" again suggesting that He is Jehovah. In verse 1:18, though, the speaker says that He once became dead and is now alive forevermore--so here we have a reference to Jehovah Who claims to have died once. Since God the Father never claimed to have died, the speaker could only be Jesus, the Second Person of the Godhead, and the One Who died for our sins.
There are many other references that could be taken directly from the JW's own Bible, poorly translated as it is, that specifically refute their doctrine that denies the deity of Christ and the truth of the Trinity.
If you need any more help with this, contact me directly at [email protected], and in either case, please let me know how things go.
Yours in Christ,
cj