Finding someone to beta read your work is not as easy as it sounds. Sure people will read it but finding people who can and will give thoughtful, helpful feedback can be tougher. Writing groups are great and all over he internet. You don't even have to leave Facebook if you don't want to. Join some groups and interact. The more active you are the quicker you will get to know other members and the quicker they will get to know you. No one likes when the person who just joined's first post is asking for something. Don't be that guy.
Once you have some people on board be sure to be specific about what you want to get out of this. Do you have a very clean late draft and just need a final run through or is it fresh off the fingers and you are looking for plot help? Do you want/need grammar edits now? Do you usually have trouble with dialog? The more you know about your pitfalls the better. As a beta reader I love to get specific directions and I am learning to give them too.
When you have a set of questions be sure to include a couple 'whats your favorite....?' it can be hard getting back a manuscript with all critical comments, but that is hat sticks out to people. Without direction they will likely point out flaws to you assuming you know whats good. Give them a little hint and you will get a good idea of what a reader likes. It may or may not be the same as what stands out to you.
Finally, and my biggest mistake, give a timeline. Make it a reasonable one, but be sure to ask people to read/critique within some kind of deadline. Without this you manuscript can end up in review limbo where you don't know if they are just reading slow, getting busy at work or have lost interest. Then you don't know if or how often you should check in and if you're like me write the hopes of feedback from that person off. I you have a deadline it sets a limit. Either you get the feedback by then or it gives a built in check-point.
Overall, being on both ends of beta reading is a great experience as long as everyone goes in knowing what is expected. Whether you're the reader or the writer ask questions, set time lines, and be open to whatever the story or comments bring!