According to Watchtower theology, the word "God" in Jo1:1 does not necessarily refer to the One we recognize as the Supreme Being
In clause 2, the word refers to the person we recognize as "deity" - "God"
In clause 3, however, evidently because of the absence of the definite article, [THE] they argue that the word "God" undergoes a shift in meaning, making it mean "A mighty one"
So, in effect, the WTS says that Jo is saying: "The Word was with God, and the Word was a mighty one"
Most scholars who have commented on this verse do not recognize this distinction, and have shown, with some justfication, that the WTS is inconsistent in the applyng of this explanation.
For a review of the JW position you might log onto the official WT web site, where you will be able to read a brochure on the Trinity online.
For an alternative point of view you could log onto Forananswer.com, or www.bible.ca/trinity.com
Cheers