Writing a book is a huge accomplishment. If you've written "The End" you know how bittersweet it is to move onto the next stage, you may even resist, but if you are hoping to traditionally publish at some point you will have to query.
The query takes a whole separate set of skills. It is, in essence, a sales pitch and many of us are not salespeople.
Nevertheless, many of the same elements that lead to an effective sales pitch will lead to a good query letter.
First is connection, and I'm not talking about the author bio. While it may be nice that you both went to the same college, or share a love of knitting, that is rarely enough to make an agent take an interest in your project. But you can make them take an interest in your characters. Presenting a compelling main character from the beginning, that they can become invested enough in to journey through your first 5-10-30 pages is a great start.
Next, is the product your selling. How is this magic sauce different from all the others? Your conflict and stakes are going to make your story stand out from the others. Show them off, so vividly they can almost taste them. Spicy!
Just like sales, you have to market your product. Who will your story appeal to? What's the word count? Where will it fit on the shelves. This is your genre, age group, word count section. Some agents like to see this upfront so do your research and make their lives easier.
Finally, I'd include the author bio. Why are you the best person to write this story/sell this sauce? Maybe here you can drop in, your story is about an elderly spinster who saves the world from alien invasion with a pair of knitting needles, and you just happen to be the resident expert on knitting needle throwing. It works!
I hope to provide you with a bunch of resources to add to your query toolbox. YouTube has a wealth, and two of my favorite are:
Alexa Donne's channel - wealth of videos and I particularly love this one on queries
How to Write a Bada$$ Query Letter
And the BookEnds Literary Agency Channel. Again, an amazing collection of great informative videos and I found this one on queries especially helpful:
The Anatomy of a Perfect Query Letter
Happy querying!
-B