Critique, like revision, is an essential part of the writing process and one that many writers dread. It's scary to share your carefully crafted words for the explicit purpose of learning what's wrong with them. But there's no way around it: if you dream of the day when thousands of people will read your book, you have to start by letting a few people read your manuscript.
It's not trivial to find readers who can give you honest, useful feedback. Family and friends may be eager to read your work in progress, but don't expect to hear more than a few (biased) compliments when they do. Most people, even frequent readers, don't have experience reacting with much more than an "I liked it" or "I didn't like it." You may be able to coax out a more detailed opinion with questioning, but when sharing with people close to you, it's often better to accept the ego boost gratefully and then look for real critique elsewhere.
Other writers are a natural choice for feedback. They're already accustomed to analyzing stories in great detail, and they understand that when you hand them a manuscript, it's not going to be as polished as a finished book. You can join a critique group or find a writing friend to trade manuscripts with one-on-one. An added benefit of mutual critique is that as you practice responding to the work of others, you'll get better at evaluating your own writing.
You may also know non-writers who have experience thinking critically about stories. Anyone in a book club is a good candidate. Or ask that friend with a book review blog or the one who has lots of opinions whenever you watch movies together. Remember that reading for critique and offering thoughtful comments takes time. If you won't be reciprocating the favor with a critique of your own, consider treating your reader to a meal while you discuss their feedback.
Some writers prefer to develop early drafts without any outside interference, delaying critique until they feel the manuscript is as perfect as they can make it. There's nothing wrong with this, but I'll argue the value of seeking feedback sooner. A critique after the first draft or the initial revision stage may point out broad plot or character problems that it's better to discover before you fine-tune your story. Just as you should expect to go through several rounds of revision, prepare for critique at multiple points and line up enough readers that you can keep getting fresh eyes.
When giving anyone a manuscript for critique, tell them what you'd like feedback on and how much detail would be useful at your current stage. If you already know about areas that need work or that will change drastically in the next draft, state that up front. You could even present your readers with a few written questions to respond to, such as "How did you feel about the main characters?" and "Was there anything that confused you?" Agree in advance whether you'll receive the feedback in conversation or in writing, on the manuscript or in a separate document. Keep it as easy as possible for your reader to do you this favor, and be sure you're on the same page (as it were). You usually can't expect a line-by-line edit; make certain your reader knows that's not what you're asking for.
Feedback isn't easy to take, especially when it's a new experience. It helps to keep in mind that every piece of criticism you hear is a chance to make your manuscript better. That doesn't mean you'll revise according to all the feedback you receive. You have to use your judgment and knowledge of your story to decide which changes are appropriate. One indication of a real problem area is that multiple readers call it out. Another sign is a comment you really hate hearing: chances are, it's confirmation of something you already suspected was wrong.
As the author of a work, you can't know what it's like to read it as someone who didn't write it. But if the intended audience is people who aren't you, start finding some of them and asking what they think. Your first readers should never be agents, editors, or contest judges, because if they are, you've wasted an opportunity by sending out work that is less than the best you can make it. When you seek and pay attention to feedback, you move toward the goal of a perfect manuscript.
More critique assistance:
→ THE WRITING & CRITIQUE GROUP SURVIVAL GUIDE by Becky Levine covers all aspects of giving and receiving feedback.