First, overcoming any Reservations you may have:
It's all about 'me me me'
You are the master of your Story. You will be able to tell things from your point of view, your side of adventures, experiences, arguments. You can give your opinion and no one can oppose you. You will have a voice. You can show the world a side of you never before seen. You can shine in your own way. What is locked in your head waiting to get out?
But I'm not a writer!
It doesn't matter how well you write, how clever you are or aren't. This is your Story and you will tell it in your own manner. The main point is making a record of your life.
A lifetime of storytelling may seem overwhelming to write. Just take it slowly - write about one event or one subject at a time. Writing a short story that covers one experience divides the project into doable segments. It will be so satisfying to watch these essays, collectively, form your Life Story.
Help! My memory is fading...
We are each eyewitness to our own life and we each remember it uniquely. So it's okay if others tell you, 'that's not how it happened.' That's how it happened in your eyes and it's your Story. They can write their own Stories, or you may want to add their version to the end of your own if the added account adds interest and color.
Should I include the 'bad' things?
Everything that has happened in your life has contributed to the person you are today. The good and the bad are intertwined in your life. Writing about the lows is just as important as writing about the highs. Some people find it cathartic to write about painful experiences. What you write may inspire or motivate others. It may touch a nerve, or make people think.
The best way to approach a sensitive or painful subject is head on. Don't let guilt or fear dissuade you. Be unflinching. Keep writing until you've finished it. Putting it down unfinished may make it hard to pick up again.
Sometimes the truth hurts...
Do you worry about writing something that will hurt someone's feelings? It's important to remember that life is well, life. It is what it is. If the subject may be sensitive to some, keep to the facts and leave judgment out of it. You may also keep things private; let no one else read them.
But to create authenticity, you should strive to always be truthful. If people read facts they know aren't true, everything you write could be invalidated.
I almost failed English 101...
Some people think they lack the ability to write a story or that their grammar and spelling skills are too poor. That shouldn't get in your way. Think of your ancestors. Would you read their stories regardless of how they spelled? Once you write something, you can have someone proofread it for you, or leave it as it is. It's the story that's important. The more you write, the better your writing will become.