first step is to write your book. Then, once it's written, find someone you trust who can edit it. Another author, a critique partner, etc. Then, once you've polished your manuscript, you'll want to do one of a couple of things, or both. Send query letters to publishers who accept unagented queries and queries to agents. If you query an agent, what you're asking is for them to take you on as a client and then they help you find a publisher.

Usually, publishers will edit also, but you must send them as clean a manuscript as possible after they've asked to see it.

For a first time author, you must have your manuscript finished first. No publisher will take you on without it finished. They can't afford that kind of investment.

I hope that helps.