Where in the Bible does it talk about horoscopes and why we should not use the stars to foretell the future?
When thou art come into the land which the LORD thy God giveth thee, thou shalt not learn to do after the abominations of those nations. There shall not be found among you any one that maketh his son or his daughter to pass through the fire, or that useth divination, or an observer of times, or an enchanter, or a witch, Or a charmer, or a consulter with familiar spirits, or a wizard, or a necromancer. For all that do these things are an abomination unto the LORD: and because of these abominations the LORD thy God doth drive them out from before thee. (Deuteronomy 18:9-12)
Although this was part of the Law to the Jewish people, the principle applies for all times. The nations that were driven out by the Jews had filled up their cup of sin, and part of that sin was trying to observe the future through various types of fortunetellers. God considered any fortunetelling to be an abomination. Perhaps that is because reliance on horoscopes, psychics, or those who read tea leaves or crystal balls shows a lack of reliance on God. If one is trusting God, he need not worry about the future.
My experience with horoscopes shows that they are vague at best and unreliable at worst. If they are not 100% accurate, they are worthless. Just reading the daily horoscope in the newspaper for a couple of weeks shows them to be unreliable. That is, unreliable if they are not phrased in such ambiguous ways as to be meaningless to start with.