As you start to write your style will emerge. Before you begin you should have some idea of how much of you will appear in the book. If you're writing a memoir nearly all of it may be written in the first person and yours may be the only viewpoint that the reader glimpses. Margaret Forster makes this change in emphasis explicit half way through her family memoir, Hidden Lives. Just before this she has been telling the story from her mother's point of view and describing her concern about her precocious child. Then the tone changes:
"It was at this time, in 1943, when I was five, that my own real memory begins, real in the sense that I can not only recall actual events but can propel myself back into them, be there again in my Aunt Jean's room-and-kitchen, standing by the window at the back of the Buildings, staring out at the outside staircase and the tops of the wash-houses, while behind me Jean asks me what is the matter ... So I can stop now, writing in the third person, stop retelling stories I was told about the years before I was born, about when I was under five, stop splicing oral history with local history and start instead letting my own version of family lore come into play. I am there, at the centre. What a difference it makes, how dangerous it is." (page 132-133, pbk)
"I" is less common in a biography - unless you want to incorporate a sense of a personal quest - but there is just as much scope to write about a person's life from different viewpoints. If you're writing about a singer you might describe how members of the band reacted to their decision to leave it or how a fan greeted the news.