Decide how long your biography should be. A business or professional biography will be short (probably from a paragraph to a page long). On the other hand, a book can be as long as you like. Determining the length will help you decide how much information should be included; a short biography needs only the basic facts.
Look at your life from the outside, as though you were someone else. This will help you write in the third person and will help you remember all of the little facts that need to be included. Think about what other people would be interested in knowing. Avoid the mundane.
Choose topics or highlights in your life that you would like to cover. The subjects need to be appropriate for the audience and situation. For example, if you are writing your biography for a web design portfolio website, you will want to focus your bio on your education, experience and accomplishments as a web designer. List life events in chronological order and don't be shy about including your accomplishments. Start writing in the third person by referring to yourself by your full name and after that refer to yourself by either your first or last name. Use the pronouns "he" or "she," as appropriate.
Avoid negativity when writing your life story. Many people avoid negativity, and if you seem to be complaining, people will be likely to stop reading. However, turning a negative situation into a lesson you learned or a positive experience because you eventually triumphed can be moving and inspirational to read.
Check your work for grammar and spelling errors. A word processing program is helpful, but you need to have someone else proofread it as well to pick up anything the program missed. This is especially the case for professional biographies, since you are trying to impress someone.