Getting a book published by a conventional publisher is a subject many people have written very much about. Here's our brief take on this complex and ever-changing topic:
The tl;dr approach to getting a publisher
Find a few books that are in your subject area, and read the acknowledgments section in each book. There, you will probably find a mention of the author's agent. Next, Google the agent to find their email address and send them a "query letter", after you Google "query letter" and find some examples, and then write one yourself. Don't give up when you get rejected; just find another agent and try again.
The comprehensive approach to getting a publisher
We don't typically endorse individual people or services, but if you are serious about publishing a book with a conventional publisher, and you want to do some research on the process, Jane Friedman is one of the best bloggers on the subject. Here is a link to one of her posts:
One thing to keep in mind on your journey to being a conventionally published author: there are scam companies out there that will charge you thousands of dollars to publish your book and that will make extravagant promises to you about your potential success. Before you get involved with any company, Google it to see if it is legitimate.
Also, you are going to have to learn a little bit about publishing contracts. Kristine Rusch in particular is a great resource for information about contracts.
Finally, please note that while there are many great reasons to publish a book with a conventional publisher, the conventional publishing process also has many drawbacks, compared to self-publishing with Leanpub. For example, you will earn a lower percentage of royalties; you will lose some independence; and you will have to sign away some of your rights to your book. In return, you might get great editorial and design assistance, credibility, and if you're really lucky, some marketing. The important thing to keep in mind with any book project is to sit down and write out an answer to this simple question: what do I want to achieve with this particular book?