When writing a biography, begin by picking an individual you are interested in. Continue by looking for facts about the person in question. Then decide what parts of the individual's life are most important, and dig deeper when looking for facts about that specific time frame. Think of what sets this person apart from other people, then proceed to write the first draft of the story. Once that is done, read it out loud, and make the needed changes.
When looking for information, start off with the Internet. However, depending on exactly who you're writing the biography about, consider using newspaper and magazine articles, biographies, interviews and history books. If you know the person, you can also interview them.
Find out which events had an impact on your subject's life. Consider their worst failures, accomplishments and contribution to society. In addition, try to find out who influenced your subject. Perhaps they had best friends, role models, family members they were closed to or teachers that had a lasting impact on them.
When writing, make sure that you draw a time line of events, so that you can include everything in the correct order. Have an outline that looks roughly like this: birth, education, personal life (marriage), achievements, failures and death.