What you choose to write depends on how you want to capture yourself. You can start with the basic stuff - where you were born, how many siblings you have, when your birthday is, etc., and move into the more dynamic stuff like what you want to do with your life, what kind of activities you enjoy, whether you are on any sports teams, your values and beliefs, your religious viewpoint, etc. etc.
Your introductory paragraph should engage your readers and make them want to know more about you. It should make the reader have new questions to ask you that you answer as you go along. To give you an example I've included below the first paragraph of an autobiography I wrote about myself awhile ago:
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"I am from a family of cowboys and cowgirls. I was born and raised in Thunder Bay, Ontario – a small city by national comparison, but the largest city in all of Northwestern Ontario with a population that fluctuates around 100,000 people. It is a rather isolated city closer to the Canadian border with Minnesota, USA than to its nearest domestic municipality Winnipeg, Manitoba. I have a great deal of pride in my hometown, and I feel a very close connection with my family; we have all been through a tremendous amount. I was born to a frightened teenage mother of seventeen, and a twenty-year-old father ready to accept the consequences of their mistake. Three years later my brother William was born. I wish I could say the rest is history, and that we lived happily ever after, but this is far from the case. Needless to say we all endured a tumultuous eleven-year separation until they put the final stake in their marriage and divorced when I was seventeen years old because my dad was ready to re-marry. Shortly after, my half-brother Michael was born to my father and his new wife – and the rest is history."