Define the audience in order to focus your biography. A mainstream audience, for example, wants to be entertained. They might not enjoy over-analyzed complexities of sibling relationships but would instead love to hear how, during a fight, one child broke an entire ceramic doll collection. If your biography assignment is to profile a serial killer for the FBI, narrow your focus to this audience's special interests. They want to know the psycho-dynamics of the family or the effects of middle-child syndrome. They want to know the personal fixations and habits of the central character.
Choose the central purpose of your biography to help limit what you include. If your purpose is to entertain, don't list every single event, patent or trophy. Prioritize and pick the most interesting and powerful incidents. In order to entertain, write your biography like a story full of dramatic tension and conflict. Balance personal reflection with ice-cold doses of objectivity. Avoid persistent, prolonged explanations. Add mystery, action and movement. If your purpose is to inform, include only those details, materials and research necessary to inform your audience of the core message.
Prioritize the important relationships. Which characters engage in conflict, tell a story, define the central character, and reveal the core landscape of the central character? Only flesh out the deeper, more meaningful characters who are essential to the story. Be tidy and efficient with the non-essential characters.
Choose and stick to a point of view (POV) to help limit subjectivity. Is this tale from a third-person or first-person perspective? To maintain objectivity or emotional distance, it might be good for the teller to occupy the perspective of someone who is observing from a different class, wealth or station. People from other stations or walks of life can create objectivity with a peculiar, stilted judgment or officious criticism of the central character that holds the person in high esteem, great hatred and petty bias. The narrator's POV becomes an emotional part of the scenes, actions and plot.
Design a dramatic arc for your story. Things can start out normal, then turn tragic or problematic. Tension can build and unknowns grow ever more mysterious. There can be modest improvements, and then setbacks and reversals come into play. Poverty, distress, desperation, the unknown, illness or something emotionally devastating can affect the potential future. Pick one over-arching story. Boil it down until it's a blitz sales pitch your agent would love hysterically.
Make the important decisions before you start writing. Of course, you may only discover these things as you begin writing, but be cognizant of how you're going to limit your biography to keep the chaotic beast under control.