You've done it! You've written, revised, rewritten, and revised again. You've gotten input from other writers and family members and scrutinized your writing until it doesn't even have meaning anymore. You're absolutely certain your book is the best it can be. Okay, this might be a little optimistic. If there's anything I've learned in my years as an editor, it's that no author is ever really done.
As a side note, if you haven't done everything listed above (i.e., writing, rewriting, getting input from others), I strongly suggest you do. A first draft of a book is just that, and your book will benefit from being worked over a few times. I promise you won't regret it.
Having said that, many authors are puzzled about what to do next. Well, whether you want to seek representation from an agent or go it alone and try to find a publisher for yourself, the first thing you need to do is write a killer query letter.
It might seem like writing a query letter would be pretty straightforward and shouldn't require much effort, but if you think that, you're absolutely wrong. Your query letter is the first, and sometimes only, impression you are going to make on an agent or publisher. If it isn't well-written and attention getting, chances are you're not going to get to the next stage, which is when the agent or publisher requests sample pages. Always keep in mind that your book is only one of thousands an agent or publisher will see in a year. Your query letter really needs to stand out if you're going to be noticed.
If you're wondering how in the world you're going to accomplish the feat of writing a perfect query letter, don't worry. There is help! There are many agent blogs that give great advice on query letters. One I particularly like is Agent Kristin Nelson's blog, Pub Rants. She has a ton of advice about writing in general and how to perfect your query letter.
Another great place to get feedback for a query letter is a writing group. I've said it before, but it bears repeating. Joining a writers' group is a great way to get feedback and strengthen your writing. Also, if your budget allows, consider going to a writers' conference. They are great places to meet editors and agents, hear advice about how to write a great query letter, and network with other writers.