A few of you have asked, so here's my answer.
How much do you need to tell an agent about yourself in your query letter? The best parts. We don't need a full bio in your query. If you've been on the best seller list five times and are looking for a new agent, put that. First.
But really, all the fluff, save that for your bio. We'll probably ask for it. The query is to get us interested. Concentrate on that.
But the question is, how much of yourself to include? I don't care if you've been writing since you could hold a pencil. I don't care if you were on your high school newspaper. BA in Creative Writing from Dartmouth? Perhaps. MFA? Could be. Leading historian on the subject your writing about? Sure, why not. You see the trend, these are relevant.
But honestly, I don't care. Some agents might. Unless you have a real solid platform, I gloss over the details. I'm interested in your book. If you can't sell your book, if you can't write it properly to get me interested, I won't care if you have won five Amazon awards. It might tip me over the edge in your favor if I'm feeling generous--I do like to see something besides you've given up your entire life for your dream to be published. Write a good query letter.
You wrote a good query letter? Cool. Then your credentials don't matter. I take a chance on writers all the time. If you can not only write a good query letter but also a full length novel without losing my interest, I don't care if English is your second language or you wrote it on napkins at a coffee house in your spare time.
That said, get as strong a platform as possible. Work on your writing career. Keep working on your writing. Take classes. Meet other authors, agents, editors. Go to conferences. It can only help you. Plus, it gives us something to pad your bio with when we try selling it to the publishers. And that's impressive. It makes it easier on us. And if you're super into marketing your own book, on or off line, that's even better.
That's my ramble for the day. First and foremost, be a writer. A good writer, if that's not asking too much. Everything else should fall by the wayside. (And note that these are my preferences. Many agents do require experience. And everyone, including myself, like to see that you are always working on your craft--it tips us off that you will be easy to work with, eager to learn, and probably won't need that much guidance.)