Write about the people who had an effect on your life. Listed below is the obituary of my Mission President, Louis Kent Payne. This was someone who had a profound effect on my life. Be sure to include pictures if you can, they make the whole experience to prepare the writing as well as to read much more pleasant.
I like to use the Roots Magic program because it gives you the option to create a book including title pages, credits as well as a photo album. However you can use any word processing program.
President Spencer W. Kimball stated that those who keep a daily journal are most apt to keep the Lord a part of their daily lives. A great deal of information can be gleaned from the past of your life by writing and keeping journals.
My great grandfather, John Paskett Stevens wrote his life story shortly before he passed away. He was able to recall memories of what life was like when his parents were fighting off the "Mormon crickets." That has been a great experience for his posterity to read about events that took place during his childhood. We can leave the same kind of treasure for those who come after us.