Okay, believe me, I know that getting that publishing contract, seeing your book out on the shelves--that's such a real, shiny goal. It's so hard to have patience in the process. We want our book to be published NOW. But let yourself take the time you need to make the best book you can. Don't rush. Keep improving, keep writing, and the rest will come. Let your absolute best writing be the first, and ever-primary step.
2. Craft your best query.
After you've got a publishable manuscript, the next step is to get your query ready. Think of your query as three sections.
First paragraph: Introduce yourself, and the give the title, genre, and word count of the book your querying. Also tell the agent you're querying why you're querying them specifically. Do they represent writers whose books are like yours? Did you meet them at a conference?
Second paragraph: This is the summary of your project. It can be intimidating, but this too can be broken down into easy parts. Four, in fact. 1) Introduce your main character. 2) What is the goal your main character is trying to achieve? 3) What are the forces/people stopping your main character from achieving her goal or getting where she wants? 4) What devastating consequence will occur of your main character does not achieve that goal?
Third paragraph: Sign off quickly with any publishing credits you already have, and possibly comp titles if you have good ones. If you're worried about this and feel like you don't have good stuff to put in this paragraph, don't stress. Just say thanks, and keep it simple.
The best tip I can give for querying is to check out the Query Shark blog. It's a big archive but well, well worth the read.
3. Strategize your agent query approach
My personal recommendation is to send queries in batches of 5-10. That way, if one batch of agents all come back with similar feedback, you can take that into account as you prepare your next batch. You can use Agent Query to find agents. Another very good way to find agents is to look at the acknowledgements section of the books that are like yours, and see if the writer thanks their agent by name. They very often do, and then you've got the name of an agent who represents and likes the kind of thing you write.
Get ready to rinse and repeat this process. Its hard in the moment to remember that rejection isn't personal, but believe me, it's not. Think of all the books you've read and loved. Only a small percentage of those books are ones you love with a FIERY PASSION, right? Books you'd be willing to read dozens of times? And that doesn't you don't love the other books you love, or that other books aren't just as good. They're just not your soul books. Agents have to feel that strongly about your book, because they're going to be working on it for a long time. So keep going. It's worth it to have patience and find the person who believes in you and your book.
4. Work on the next thing.
This has been one of the best things for me to keep in mind at every stage of the publishing journey. STARS, my debut book, is my fourth. I worked on it while I was querying my other books; books that will never be published, and rightly so. With each project, we grow as writers, and that's exciting. So learn to finish. Then send out your project as determinedly as if it's your only project and your career depends on it, and then start work on the next thing as if its your only project and your career depends on it.
Keep going! This takes years of work, grit, and patience. It takes humility and constant learning. Find friends and mentors who can help and guide you on this process. Stick to it, and good luck!