What a birth chart actually is
A birth chart is a snapshot of the sky at the exact moment and place you were born — where the Sun, Moon, and planets were, which constellations sat behind them, and the angles they formed to one another. Astrologers read it the way other people read a map of a city: as a layout you can move through.
Your chart needs three things to be drawn: a date, a time, and a location. The date plus time gives the sky's exact configuration. The location plus time gives the houses — the twelve life-domains the planets fall into. Without a birth time we can still calculate your Sun, Moon, and most inner planets, but the houses and Rising sign won't be reliable.
How to read what comes back
The big three — Sun, Moon, Rising — are the first thing to look at. The Sun is the identity you grow into. The Moon is the emotional weather you live inside. The Rising sign is the way you arrive in a new room. People who say they don't feel like their sign often have a Moon or Rising doing the actual day-to-day driving.
After that, the inner planets — Mercury, Venus, Mars — describe how you think, what you love, and how you go after what you want. The outer planets, Jupiter and Saturn, draw the longer arc: where life expands and where the work is. The lines crossing the inside of the wheel are aspects: the conversations the planets are having with one another.