The key to every detail in your bio is: Will it be meaningful—or perhaps charming—to the agent/editor? If your bio can reveal something of your voice or personality, all the better. While the query isn’t the place to digress or mention irrelevant info, there’s something to be said for expressing something about yourself that gives insight into the kind of author you are—that ineffable you. Again, charm helps.
Here are the categories of most pertinent info for your query bio.
Be specific about your credits for this to be meaningful. Don’t say you’ve been published “in a variety of journals.” You might as well be unpublished if you don’t want to name them. If you have no fiction writing credits, you don’t need to state that you’re unpublished. That point will be made clear by fact of omission. If you have a long publishing history, just list the ones you’re most proud of or the ones most relevant to what you’re pitching. Unless you’re pitching nonfiction I don’t recommend including academic or trade journal credits, since they don’t convey storytelling ability.
Lots of people have self-published, and a self-publishing history doesn’t hurt your chances with a new, fresh project. However, if you’re trying to get an agent or publisher for a book or series that’s already been self-published, my advice is to not bother trying. (If you must, here’s how to pitch an agent with a self-published book.)
Do not make the mistake of thinking your self-publishing credits make you somehow more desirable as an author, unless you have significant sales, meaningful awards, or other notable success. In that case, you should probably mention your self-publishing history, offer a ballpark total sales figure, and proactively share how many customer reviews you have at Amazon/Goodreads, along with your average star rating.
If your career lends you credibility to write a better story, by all means mention it. And it’s OK to mention your profession more generally, as agents or publishers will be curious about your background. Usually one to two sentences is plenty.
Avoid commentary that might be arrogant or off-putting. For example, this kind of statement does not help: “I’ve never tried writing seriously since I’ve spent the last 50 years practicing medicine, but I’ve just finished my first novel about a doctor that shows what the job is really like, and I think it will be a bestseller.”
Teachers of K-12 who are writing children’s/YA often mention their teaching experience as a credential for writing children’s/YA, but it’s not, so don’t treat it like one in the bio. (Perhaps it goes without saying, but parents should not treat their parent status as a credential to write for children either.)
Mention any writing-related degrees you have, any major professional writing organizations you belong to (e.g., RWA, MWA, SCBWI), and possibly any major events or workshops you’ve attended to help you develop your career as a writer. If you’ve had mentors that the agent or publisher would recognize by name, mention them. (But do not include any subjective endorsements or words of praise in the query.)
I wouldn’t talk about starting to write when you were in second grade. Or how much you’ve improved your writing in the last few years. Or how much you enjoy returning to writing in your retirement. Do mention anything that might prove your seriousness and devotion to the craft of writing if you can. If unsure, leave it out.
If your book is the product of some intriguing or unusual research (you spent a year in the Congo), mention it. These unique details can catch the attention of an editor or agent.
Most writers should not mention awards or competitions they’ve won because they are too small to matter. If the award isn’t widely recognizable to the majority of publishing professionals, then the only way to convey the significance of an award is to talk about how many people you beat out. Usually the entry number needs to be in the thousands to impress an agent/editor.
If you have no meaningful publication credits, don’t try to invent any. If you have no professional credentials, no research to mention, no awards to your name—nothing notable at all to share—don’t apologize for it. Perhaps say something brief about yourself—where you live, your education, your day job, hobbies. Remember: Even if you’re unpublished, you’re still completely respectable. For fiction, you’re mainly getting judged on the story premise, not your bio.