Hermione Lee's biography is fantastic and highly recommended but be warned that it is loooooooooong.

Woolf is so interesting on every level, and she left behind extensive diaries and letters so that we know basically what she was doing every single day of her adult life. Which is great, but also makes for a very, very long biography. So I don't blame you if you don't want to read it, but I thought I'd throw it out there just in case.

If you're wanting to start with her fiction, I'd start with Mrs. Dalloway.