How to write an autobiography?

I have decided to go ahead and write an autobiography. No my ego has not inflated to that point, yet. I am just interested in writing it as a kind of time capsule for my own sake, so I can look back on it in future. I am coming up to 30 years of age in January, and figured now would be a good time to do it.

Are there any general rules when doing this kind of thing? is there a specific style, way or method to an autobiography?

While I do read a lot, I hardly ever read stories, and have probably never read a full published autobiography in my entire life. So even thought I read way more books than average, I have no experience in this kind of writing.

Are there any pitfalls I should look out for? Have you got any general advice?

How should a person write their autobiography?