In my day job I spend a lot of time reading the writing of others and instructing students in "how to write" One of the biggest things I tell them is to, whenever possible, have someone else, or preferably multiple someones, read their work before submission. The same holds true in the publishing world. Beta readers, as they are sometimes called, can do wonders for your manuscript.
What does a Beta Reader do? Well some of that is up to you, and them. Basically a Beta's job is to read your work and critique it. They can look at grammar, spelling, plot, characters, anything you need help with. They tell you what is working and what area's you may need fix. The great thing about a Beta reader is that they are not as attached to a piece as you are, so they are more likely to catch that a section should be cut or added to.
How do I find one? Just ask! Most of my Beta's are other members of my writing group, including the other ladies of PUB518. I have also had family members read. If you do not feel comfortable asking someone close to you, reach out to groups online. Many groups on facebook, reddit, and other sites, exist for the sole purpose of helping other writers.
The main thing to remember when using a Beta reader is to remember that they are trying to help you. Some of what they say may be difficult to hear, especially if you have been working on a project for a long time. But in the long run they are trying to make your work better.