Mormon doctrine is basically polytheistic. Jesus is A God but not THE God the Father of the OT. There are in fact many Gods. God used to be a man like us, and we can one day be like Him and have our own little planet to 'rule'. Jesus and Satan are spirit brothers.
God the Father has a physical, flesh and bones body. To make Jesus he had sex with Mary (this is one of the more odd doctrines and one some mormons will not admit to, but it is there to be read).
This is just a sampling of their strange doctrines. They also teach that the 10 lost tribes of the Jews actually came to the western hemisphere and that the native americans of north and south america are their decendents - despite the fact that we can scientifically show that none of these native peoples are semitic in origin. Jesus of course came to this hemisphere to preach in the Americas.
The entire story of how Joseph Smith 'discovered' these truths is quite a story in itself.
To the original posters question of how to 'preach' to them, it is a difficult thing. Mormon outreach is not my specialty. But I will say that I know that in general it is (like reaching, say, muslims) a longer process you must take a view of, and you will in most cases need to do the work relationally.
They do not use our version of the Bible, theirs is called the New World Translation, and it is frought with errors in translation, most famously in the first chapter of the Gospel of John. Unless you are well versed in the issues of the translation, and the original language in those areas though I would not bring this up, as they will argue.