Will they work for me?
This is huge. If we are putting out work out there for people to read we want to make sure that it's perfect. I know I do. One key part of beta readers is that they give you feedback. You have to be receptive to that. Not everything that they say is going to be nice. They are going to have critiques. You have to have a tough skin. They are only saying it to make your work better.
Point two, listen! You have to listen to what the readers are saying. These people are giving you their time for free to read the book. They take their time to edit your piece. Listen to what they say. Take their critiques and use them.
I know this might sound weird but you have to want your work to get better. I know sometimes I write just for fun. I don't publish it so I don't want anyone else to critique it. Sometimes that's okay. But don't get a beta reader to edit it. You have to want your piece to improve and get feedback.
Where do I find one?
Most of the time people have three or four beta readers. Each person has different opinions and that helps a lot. But where the hell do you find three people to critique your book for free? Well, one beta reader could be a relative or a friend. Be careful though because sometimes they are too nice! They are afraid to hurt your feelings. There are also tons of online writing groups. All you have to do is google it and hundreds will pop up. Look for a similar genre and join!
Social media is so huge right now. Google+, twitter, Facebook, tumblr. All these sites you can use to your advantage. Put out a note saying that you need some help. More likely than not, someone will respond willing to help out.
Another good way I find people to talk to about writing is in my classes. Of course I am a college student so I have access to classes but there are other ways. There are tons of workshops going on out there that can help. Just ask around.
For me, I love to read. If someone asked me to read a book for them and give feedback I would do it in a heartbeat. It's a free book to read. Why not help out a fellow writer?