Writing a book is hard, and it's not really something you can do in complete isolation. At some point someone is going to have to read it and if you're wise you'll look for objective and experienced advice to point you in the right direction.
But where to find it? Your first port of call for free advice is a peer review site. There are many and they are as good or bad as their members so you have to suck it and see which one is best for you. Any site that allows anyone to join will have the widest range of ability, and therefore the strike rate for good crits is going to be lower. Having said that, you can usually glean something helpful from any crit. Even a non-writer will have a gut reaction to your story that's useful to know. With a little time and effort you can build up a picture of how your book is being received.
And the great thing about peer review is you learn an awful lot about what works by reviewing other people's work. The problem with your own manuscript is you're too heavily involved with it and therefore can't see the faults. That isn't a problem with somebody else's baby. The flaws are much easier to spot, and reading critically teaches you to read your own with a dispassionate eye.
But peer review has limitations, and sooner or later you may want to find professional advice to help you reach the next stage. I've never paid for editorial advice, and I would guard against throwing your money at just anyone. With the cuts in publishing there are more freelance editorial advisors around than ever, and some are better qualified than others. Personally I wouldn't take advice from anyone who hadn't reached a significantly higher level in the publishing business than myself. Another wannabe, however perceptive and well intentioned, and cheap, is probably not good enough. So if you are tempted down this road, make sure you go for someone with a proven track record either as an author or an editor of published books.