As every writer knows, beta readers are an important part of the editing process. I think you shouldn't choose (or ask) someone to be a beta until you are as close to ready as you can tell--that is--when you are sure you are done editing.
I think you should get at least two beta readers. I think you should find one person who likes the genre you're writing in and one who is just simply good with the English language. That way, you have one person that will tell you if they simply like it, and one who will let you know about any major boo-boos you may have missed.
When you find a couple beta readers don't pressure them into a thorough review. Unless you're surrounded by other writers on a daily basis (i.e. working at a newspaper) then the person reading it won't know how to do a formal review. Be curteous, let them know that you'd like a general idea of what they liked and didn't like about your story, but you'd be fine with them just giving it a rating, like 4/5. Of course, if they're willing (and this is mainly where the good-at-English-stuff beta comes in) let them know it would be more helpful to know what needs to be changed. To make notes, and references. One other thing, let them know how they can get a hold of you if they need to ask you anything. Email, phone, facebook, myspace, twitter, whatever. Make sure they know that if they want to grill you about the plot, you will be available. And don't be offended if they call you at three in the morning! I'd see that as a huge compliment if your story kept them up into the middle of the night to finish it.
Have a list of questions ready before you send your story off to be read. Make sure they are simple things, nothing that will confuse them or make them reluctant to give answers. What did you like about it? What do you think could be different? Do you remember the MC's name(s)? Was it believeable? Favorite part? etc. etc. Make notes if you're doing this in person. Know that even though you're extremely nervous to hear what they're going to say, they could be just as nervous. If you're asking your best friend, they may be worried they'll disappoint you. When you've finished asking all your questions see if they have any. They may want to know if you plan to do a sequel, or what happens to a secondary character. Don't forget to thank them. These people have just done you an amazing service.
One final thing. You may be tempted to offer them a copy of the book when/if you get it published. Maybe even a signed copy. Granted, there are people that will be excited by the prospect, but you shouldn't be the one to ask. Its a bit pretentious. If they want one, they'll ask. Whether you say yes or no is up to you.
Well, that's it! After all that work, its time to find an agent or a publisher. And that--that's a whole other diemma for a writer.