Do you have critique partners or beta readers? Get someone who is not a close friend or family member to read the entire book. Your ideal readers are either other experienced writers or readers who love the genre you write. If you have readers who have never done this before, you can give them a simple questionnaire to fill out asking about the concerns you have regarding plot holes, character development, etc., but don't give it to them until AFTER they've finished the book.
Start with by getting some feedback. Once you have that, you can develop an editing plan. Before you think about hiring an editor, you should clean the prose up, polish the story, and bring it to the best level you know how. That way, an editor can focus more on structure and development and less on grammar, spelling, syntax, and other nit picky problems.