1. When you first start writing your story, just write it and get it down, worry about editing later, and don't get bogged down in the details of grammar or word choice.
2. That being said, edit, rewrite, and edit for flow, inconsistencies, and style.
3. When you feel you are finished and ready to publish, get a professional editor, not your sister who is an English teacher or next-door neighbor who is a prolific writer but a professional, they are worth every penny.
4. Find an editor that enjoys your genre and ask questions about their editing, what you expect and what they do may not be the same. Some do line editing, copy editing, developmental editing, etc. Make sure you ask for what you want and match with the right editor.
5. If you are a first-time writer with no writing experience, you can send query letters and see if anyone bites but it's very hard to find an agent. Don't be disappointed or give up. You had a story that you wanted to tell and if you can't get an agent, it's not the end of the road. Self-publishing is a respectable way to get your book into the reader's hands.