Every planet through every sign.

A planet in a sign means that planet was located in that zodiac sign at the moment your chart was cast. The planet describes the function (Mars = drive, Venus = relating, Mercury = thinking) and the sign describes the style (Aries = direct, Cancer = protective, Libra = harmonising). Combined they tell you not just what part of you is at work but how it tends to express. Sun in Aries means your core identity expresses with Aries qualities; Mars in Cancer means your drive expresses through Cancer territory.

Use a birth chart calculator with your birth date, time, and place. The calculator looks up where each planet was at that exact moment from a standard ephemeris and tells you which sign each one was in. Sun sign you can usually estimate from your birth date alone; Moon, Mercury, Venus, Mars, and the outer planets need the actual ephemeris. The free birth chart tool here computes all of them precisely.

Astrology has the concept of dignity — some planets express most fluently in certain signs (their domicile or exaltation) and most awkwardly in others (their detriment or fall). Sun in Leo and Mars in Aries are domiciled (the planet is at home); Mars in Libra is in detriment (running counter to its nature). But 'better' is misleading — every placement has gifts and tensions, and an ostensibly difficult placement often produces more conscious work and more interesting people than an easy one.

Mercury and Venus are inner planets — they orbit the Sun closer than Earth does, so from our view they never appear far from the Sun. Mercury can be at most 28° from the Sun (so always in your Sun sign or the one adjacent on either side); Venus can be at most 48° from the Sun (so within two signs of your Sun). That's why most people's Mercury and Venus signs are near their Sun sign rather than evenly distributed across the zodiac.