Astrologers believe that the planets at the moment of your birth have key influences on your personality. In order to calculate your Natal (or birth) Chart you will need to know the Time and Place of your birth. Most people know their Sun sign (the position in the zodiac that the Sun was in on your birth-date. The position of all the planets is described in your Natal Chart. You will also need an Ephemeris to help calculate this.
Most systems use Greenwich Mean Time (GMT) as a basis of calculation.
In some countries your birth certificate or hospital records will tell you the time of birth. When you were born, each of the planets in the solar system had a precise position in the zodiac. Your Natal Chart reflects these positions, and the interpretation of your chart yields information about your signs and astrological houses and their influence on you.