There are many different ways to get a book published but the traditional route is to find an agent, submit a proposal or a draft, and then find a publisher.
Here are some tips:
When looking for an agent check they are accepting submissions and approach agents who represent authors you like, or who your work might fit well with.
For non-fiction books it’s enough to submit a proposal. This might include a sample chapter, a list of possible chapters and a suggestion of where your book might fit into the market.
Be open to editing and suggestions. It’s a great privilege to have your work edited by a great editor but it’s also important to have someone you trust.