I would suggest that you think carefully about the parts of your personality or life that you want to convey to your readers the most. Something that is meaningful to you that gives your readers a glimpse into who you really are. Whether you begin with a quote that is significant to you, or you share an anecdote from an important part of your life, you can use your opening line to connect with your readers from the very beginning of your book. It does not have to be anything profound, but it should be honest and true, straight from the crux of your being.
I can understand the difficulty of your task. Writing an autobiography is not easy. Part of the reason for this is owing to the fact that there is no set way to write one. You can pretty much start in any way you like. Hence, be creative. In light of this, probably the best way I can help you is to give you a few examples of great autobiographies in history.
First, we should start with the one who created the genre - Augustine. Here is how he starts his Confessions:
"Great art thou, O Lord, and greatly to be praised; great is thy power, and infinite is thy wisdom."
Since Augustine was a Christian bishop, he wrote from a religious point of view.
Second, Mark Twain's autobiography is more down to earth. He starts with these words.
"I was born on 30th of November 1835, in the almost invisible village of Florida, Missouri."
Here is how Malcolm X's autobiography starts:
When my mother was pregnant with me, she told me later, a party of hooded Ku Klux Klan riders galloped up to our home in Omaha, Nebraska, one night.
As you can see, there is no one way to start. There is no formula to follow. So, be creative and tell your story. There is no story like it, as we are all unique.