One of the very first things I would do is join Biographers International Organization (BIO), which is the only literary organization devoted solely to biography. BIO members can help answer a lot of your questions, and (if you're a member) you can download off BIO's website copies of proposals for successful biographies (including, among many, my proposal for
Jim Henson). There's also an annual conference (in Richmond, Virginia in 2015) with all sorts of useful panels and plenary sessions, along with knowledgeable speakers (including Pulitzer winners, Guggenheim recipients, and so on). It's a great resource for biographers.
BIO's home page is here:
http://biographersinternational.org/ and you can join here:
http://biographersinternational.org/join/
And yeah, I'm the current president of the organization. But I wasn't when I joined in 2009 -- in fact, I was still unpublished. Now I've got two bios published with another in the works.