Your astrological sign may not fit you because it might be wrong if you were born on the cusp. Most of the time, the generic sun sign readings do not fit the person, because there is so much more to us than one planet. A natal chart takes into account the following: planets in the signs, planets in the houses, aspects between planets, retrograde planets, Ascendants, Midheaven, Nadir, Descendant, the moon's nodes and comets depending on the astrologer.
A Natal chart tends to be very specific about the person. In order to get the most accurate astrological reading you need the time, date and place of birth. A reading can be done without the time, but it is much less accurate.
I'm an astrologer, and have been studying astrology for over ten years. I find it fascinating.