First think about who will be reading this. A blurb published in a magazine with your story is a way to touch base with your readers. They will be interested in fun information, and also how to find your other works. An editor or agent reading a query will want to know about your published work, publishing experience, and anything relating to the book you're pitching them. If you are a speaker at a conference, and you need a bio for the brochure, your readers will want more formal information about your background and qualifications.
Also consider how long it should be. It's a good idea for any author to write several biographies, of different lengths, because you never know when you might need one.
For book and magazine blurbs, the editor should tell you the length, but if they don't, keep it SHORT. Especially for author bios in magazines, where it should be twenty-five words tops. (For these really short bios, think hard about step 6.)
For a query letter, maybe fifty words. The writers biography part of a query letter should be about a quarter of the letter. Less if possible, more only if relevant to the book you are pitching.
A longer author biography for a conference or press release can run to several pages, but you are better off keeping it under a page -- maybe 100-250 words. Keep it short unless you have reason to believe you should do more (if they ask you to, or you see that all the other writer bios are really long). If you want to do more material for a press release add an "interview", which can be several pages long.
When I don't get any length requirements, I sometimes write two bios and give them a choice. (Be careful with that. They inevitably choose the one you like least.)
Make a list things about you that relate to the story. What got you interested in the subject of the story? What got you writing? Where did the idea come from? What have you done that makes this story, book or article personal to you?
Try to focus on one thing on this list, but you can sometimes tie several of them together into one. For instance, when I wrote a mystery/western, I wrote "Camille LaGuire spent her childhood riding horses, watching westerns and reading mysteries. Of course she ended up writing gunslinger whodunnits!"
These personal details are most appropriate for blurbs and some longer biographies. For query letters, you want to be casual, but only use this sort of detail if it really reflects the nature of the book. You want the whole letter to sell your style, in that case.
For queries and long biographies, make a list of your credits. (In a very short magazine blurb, don't waste time with this. Better to use that space to give them your website, where they can find all your writings.)
Pick no more than three credits to highlight in your bio. Choose them by whether they are relevant, prestigious and recent. Consider again your audience - what would this audience care about?
Be careful about resume type information (school, work experience, volunteer experience). Such info tends to be pretty boring, and there are only a few cases where you want to use them. They may be necessary as "credentials in a conference bio or press release. In that case, wow them with your best stuff, and keep it short, and keep long lists to the END of your bio. The other case is if it is relevant to the article, book or story you are promoting. If you write about the Civil War, people will want to know that you are a professor of American History who got your degree an ivy league university.
The other exception is if you are truly passionate about one of these resume items. It does not belong in a blurb or query letter unless it relates directly to the story in question, but in a longer biography, especially one for a press release, it's a detail about who you are. Don't try to force it in where it doesn't fit, but by all means, if you are passionate about working for Habitat for Humanity, or making hand-crafted door-bells, mention it as a personal interest item. Relate it to your writing work if you can, even if it is only that the writing is a break from saving the world.
Consider what action you want the reader to take. For instance, you want your fans to be able to find your other works, so your bio on a story or book should include your website address. Your author biography on a conference brochure should entice people to come to your presentation so you might mention the topic of the presentation in your bio. And you want an agent or editor who is reading a query letter to read your book, so keep the stuff about yourself to a minimum, and push the book instead.
Now that you've considered all of this: write your bio. Use step 1 and 2 to figure style, and steps 3-6 as material for the content. Make sure you use the right pronouns. For published material (blurbs in magazines, books, press releases and brochures) write in third person. "Susie Scriptor has written fifteen stories for children and adults." In a query letter, you should write in first person: "I have written fifteen stories for children and adults."