The First Council of Nicaea met with one main purpose - to deal with the crisis caused by Arius of Alexandria. Arius was a presbyter in Alexandria, Egypt, who objected to the teaching of the bishop of Alexandria, Alexander. This controversy fired-up around 318, when Arius objected to a sermon in which Alexander taught that the second Person of the Trinity - the Son, was eternal. In Arius' mind, this kind of theology would lead to two ultimate principles in the universe, and so a kind of dualism instead of monotheism.

Alexander moved to have Arius deposed, and he left Egypt with a number of supporters. Arius then wrote letters and attempted to garner support for his position, which he was somewhat successful in doing. This, more or less, pushed Alexander to do likewise - to broaden the conflict and involve other church leaders.

The controversy grew large enough that it was causing significant division in the church, and Constantine having recently taken full control of the empire moved to intervene. This he did, organising a small council at Antioch in 325, before the larger council of Nicaea 325.

Between 200 and 320 bishops attended, based on various accounts; almost all were from the Greek-speaking parts of the empire. Its main product was the Nicene Creed, which you can read in translation here. Of particular note are the anathemas that follow creed - all aimed at certain aspects of Arius' teaching.

On this issue of the divinity of Jesus - the answer is "kind of". Nicaea didn't decide Jesus was God, it attempted to clarify what Christians meant by calling Jesus 'God'. For Arius and his followers, 'god' as a non-unique title given to Jesus, and he was not to be identified with the one, sole, uncreated, Almighty God of the Universe. But they still called Jesus 'god'. The fact that we 21st century English-speakers think that there is a category gap between God and 'everything else' is a consequence of the fourth century theological debates, not a presupposition.

The Council also did not decide the composition of the Bible. In fact, Nicaea said nothing about this. Dan Brown is factually incorrect and misleading on this topic. It did decide a number of other church-matters, which you can read here in translation.

Sources:

Hanson, RPC, The Search for the Christian Doctrine of God: The Arian Controversy 318-381. Edinburgh: T&T Clark, 1988.

Ayres, Lewis, Nicaea and its Legacy: an Approach to Fourth-Century Trinitarian Theology. New York: OUP, 2004.

Behr, John, The Nicene Faith, 2 vols. Crestwood, NY: St. Vladimir's Seminary Press, 2004.