Read up on your market. Young adult (YA) literature has changed a lot over the past decade, and keeping up with the latest trends is as important for YA writers as it is for teens. Haunt the YA section of your local bookstore to see what's hot. Find books that are similar to yours and scrutinize their content, length and style. Add the publisher, editor and agent's names to your little black book, so you can submit to them when you're ready.
Research agents, editors and publishing houses that specialize in young adult literature, and consider which ones would be a good fit for your manuscript. The site Agentquery is the happening place to pick up an agent nowadays, and the Writer's Market is a trusty old standby (see Resources below).
Write an attention-grabbing query letter: a paragraph or two about your story, a paragraph about yourself and a paragraph about why the person you're querying is the perfect match for your manuscript. Agents and editors slog through countless numbers of query letters every week, and it's your job to make yourself stand out. Just make sure it's your brilliant writing that gets you noticed, not affectations such as writing on purple paper or putting stickers on your stationary. Literary types are conformists when it comes to manuscript preparation---dress your story in a weird font, and they'll freeze you out of the party.
Begin submitting your query letter to the list of agents and editors you've compiled. Adhere to the agent or editor's submission guidelines exactly; you're on their turf, and they get to call the shots. Usually this means printing your materials in 12-point Times New Roman font on plain white paper, single-spacing your query letter but double-spacing everything else, and including an SASE---a self-addressed stamped envelope---so they can mail you their response on your dime.
Never give up. The vast majority of authors were rejected numerous times before getting a book published; the difference between them and everyone else is that they didn't give up. So don't be a drama queen and wallow in rejection. Just keep submitting your manuscript until it finds its rightful place in the publishing world.