I am an astrologer and at one time in my life wrote an astrology column. I hate to say it, but "Sun Sign" horoscopes have very little to do with real astrology, which like Nicole mentioned, involves a study of the heavens at an individual's exact time and place of birth. There are many planets involved and each person's astrological chart is quite complex and unique. I find it fascinating to study! I wouldn't say that there is nothing valid to your Sun sign or the horoscopes that you read, but would only warn you to keep in my mind that it is a mere fraction of a very big picture. Sun signs are a very important part of one's astrological chart, however so is the Moon sign, the planets, the Rising sign, the houses, etc. To give you an example, I am a Capricorn, but my Rising sign happens to be the opposite sign of Capricorn, which is Cancer. Therefore when reading my Capricorn horoscope, while some of it may be true (in terms of my Sun), much of what my Cancer horoscope reads would cancel out or contradict my Capricorn forecast. On the other hand there are people such as my father who have an unusually high amount of planets in one sign. In his case, reading the horoscope for that sign will be more accurate. But even still, dividing the entire population up into 12 personality types leaves much to be desired.
Hope that helps!