I asked a JW once about this text and why they translated it "a god" and his reply was that the Bible speaks of many gods. I think he had in mind the verse from 1 Corinthians 8:5
For though there be that are called gods, whether in heaven or in earth, (as there be gods many, and lords many,)
To which I reminded him that this was a reference to false gods. Was he saying then that John was calling Jesus a false god? He didn't answer. But the JW argument goes something like this, "If Jesus is called the SON of God, how then can he be God as well?" My question in response to that is usually, "Jesus also called himself the son of man. Does this mean that he he therefore cannot be man?"
It is well to realise also when speaking with JW's the purpose of John's gospel. It was partly a response to the gnostics of his time who denied that Jesus was come in the flesh. Therefore the heresy that the early church had to contend wiith was not that Jesus was not God, but whether he was indeed MAN.
So John was not concerned so much to prove that Jesus was God. Which makes these verses in John chapter 1 even more remarkable. For we read that he was indeed THE WORD of God and as the word of God was of necessity God himself. For how can we conceive God's word as being separate from God? Just as I cannot conceive my words as being separate from myself. My words are part of me.
Jesus therefore is the WORD MADE FLESH. God's word made man. The divine coming into the world of man and clothed in flesh as man. He is the very heart and mind of God Himself.
The truth is that the so called Jehovah's witnesses cannot get round the fact that however they translate John chapter one they cannot separate the WORD from GOD. For however you translate it, it must always literally be translated "The word was God". To translate it "the word was a god" is a bad and clumsy translation. Anyway, where else in the New Testament do you find Jesus referred to as "a god"? For there is but One God. Of course the doctrine of the trinity cannot be understood. Someone said once, "What man invents, man understands". Be sure that no man invented the Trinity. The fact of three persons yet one God.
ps: It is also useful to remember this when talking to JW's. Their name is taken from the OT text in Isaiah 43:10
Ye are my witnesses, saith the LORD, and my servant whom I have chosen : that ye may know and believe me, and understand that I am he: before me there was no God formed , neither shall there be after me.
I always turn to the NT words that Jesus spoke to his disciples in Acts 1:8
But ye shall receive power, after that the Holy Ghost is come upon you: and ye shall be witnesses unto me both in Jerusalem, and in all Judaea, and in Samaria, and unto the uttermost part of the earth.
The reason why Christ's witnesses received the nickname "Christian" was`because`they were always witnessing for Christ. Always talking about him. JW's on the other hand gave "themselves" that high folluting name.