Because of our 23.5o precessional tilt, Earth's equator is not lined up with the rest of the Solar System! There is a second great circle on the Sky, the Ecliptic, which defines the path of the Sun among the stars on the Celestial Sphere. It is tilted, quite naturally, 23.5o compared to the Earth's Celestial Equator. In fact, all the planets too, are seen to lie along this path (except Pluto). This is where your `sign' comes from. The Zodiac is made up of the 12 constellations which the Sun passes through on its year long trip along the Ecliptic. When you are born, the constellation that the Sun is presently projected against becomes your 'sign'.