There are several ways you could go with this. Since an autobiography is inevitably written while you are still alive, one possible idea would be to write a sentence that states what your future plans will be based on what has happened to you in the past. You might say something like, "After all the excitement I have experienced so far, I look forward to my future life in my quiet cabin in the woods," or, "After wrecking three cars, I have taken up building bicycles: maybe one day I'll pedal past you on the road."
Another method involves pulling up famous autobiographies online and using another person's last sentence as a model. Imitation is a tried and true method of learning to write—and don't worry, your life is your own, so what you write will be your own too. I pulled up Helen Keller's autobiography and have copied the last two lines below:
Thus it is that my friends have made the story of my life. In a thousand ways they have turned my limitations into beautiful privileges, and enabled me to walk serene and happy in the shadow cast by my deprivation.
Friendship is an important theme in her life, especially her life-changing relationship with Anne Sullivan, and her last sentence reflects that theme. It sums up how friendship has helped her. You could do the same with a major theme from your own life story.
Autobiography is a form of narrative dealing with the self- it tells one's life story or recounts important events and themes up to the time of writing. Because autobiography is self-written, most people are writing well before their life story has really come to a close! I think that this leaves you with an interesting choice when it comes to writing the ending of your autobiography...
Do you want the ending of your autobiography to present finality, continuity, or change?
For example, let's say an author working on their autobiography has had a long career as a chef. In ending their autobiography, they could imply a sense of finality by saying that they are retiring or closing their restaurant. This "closes the book" on the major theme of their career as a chef, and even though they may do other things in their retirement, this part of their life is over.
To present continuity, the chef might end their book by saying that they look forward to many more years working in their restaurant and do not plan to make any new changes. Perhaps their autobiography has recounted their rise to success, and now their life has entered a stage of stability.
Alternately, the chef might mention that they are opening a new restaurant, or are moving to another country to learn to cook foreign cuisine. This presents change in the story while opening up the possibility that their life story will continue to evolve.
How might the options of finality, continuity, and change relate to your personal autobiography? Consider whether you are ending a theme or "chapter" of your life, continuing in a theme, or making some changes. The most appropriate ending should reflect information already established in your autobiography, but tell the reader what they can expect to happen next- even if it is nothing at all!
When you write about yourself (autobiography) you want to focus on a snapshot of your life; a very pivotal moment or moments that would make a great story. I often tell my students who write personal essays to not give me a play by play of one event after another from birth till present, but give me a few significant events (a snapshot) and tell it with great description.
If I were writing an autobiography I would start in the middle of a scene of something really exciting that had happened to me; the time I caught a shark or hit the home run to win a game. I would talk about how it impacted me--helped me learn determination and battling against the odds. I would then talk about similar challenges in my life that I have had to overcome. Then my final sentences would be a toss back to those big moments in the early days of my life (the shark and the home run).
To summarize: I would begin by telling an exciting moment; grasp the reader's attention. Tell all about it and its impact on your present life. I would tell about the lessons you've learned from those moments and conclude with a sentence referring to the exciting moment you began the story with.