It can take some time to find the right editor, so you may want to start asking around even before you have a text that is ready to send. If you know a fellow author who can recommend someone, that’s wonderful, but if not, it’s fine to poke around the websites of some editors and see who appeals to you (and seems to work within your genre). Then contact them by whatever method they request (often a contact form on their site).
Obviously there is a big difference. I actually started with screenplays (with some mild success) partly due to hearing that as long as you have a good story, the grammatical errors won’t be a big deal. Well, that was not true. Maybe if a producer liked the story or I was already established but every contest I entered always came back with feedback about the grammatical errors. Even if I won or placed as a finalist, I always received critique on that. Very little is spelling error, it was more about structure, etc.