Find a literary agent. Aside from its DAW Books imprint, Penguin doesn't accept unsolicited manuscripts from authors. Visit the Agent Query website and search for literary agents who represent your book's genre. Query all of the agents on your short list---ensuring that you follow their exact instructions---until you find a good match.
Prepare your submission. Work together with your agent to make sure your manuscript, synopsis and proposal are perfect. You get only one shot at impressing a Penguin editor.
Ask your agent to put the Penguin Group at the top of her list of publishers to whom to submit your manuscript, and keep informed of your progress.
Once you have an offer, read Penguin's contract carefully, and reasonably negotiate terms. Have your agent check all rights clauses before you sign.
Sign the contract and wait for the next step---usually working with a Penguin editor to revise problematic areas in your manuscript.