Do you run your diary? If not, nothing is lost. Without Grounds of how old you are, describing your experience can be an interesting practice, which in the future will pay off.
Entries to your diary you can enter every day, every few days, once a week or only when happend something extraordinary, it depends on you. It is possible that having a diary will motivate you to keep more interesting life, even if it is already quite interesting.
His own diaries led famous people like Isaac Newton, Einstein, Thomas Jefferson, and much more. Interestingly, a study showed that people who keep diaries, and observe their thoughts, gain more control over their life and are more intelligent. You can read more about it here.
To get started, all you need is a thick notebook, so that did not end too quickly. It is best to buy some of the same notebooks, then keep continuity even if the previous is complete. Another option is to buy a binder with the ability to attach the cards. Hide your diary, so you are one person who know where he is. This way you will be able to be completely honest recounting the events of your life.