It’s not a question I get very often, but when I do this is a useful outline for explaining why I believe in the deity of Christ. It usually comes up when a student taking a religious studies class is told that the earliest Christians didn’t think that Jesus was God — this belief is alleged to have been invented out of whole cloth centuries later. Total poppycock, but widely believed in some circles.
In case you need something broader, check out The Biblical Basis For the Doctrine of the Trinity by Rob Bowman. It’s an outstanding guide to the Christian doctrine of God existing in three persons.