First, in the US and most countries, you get a copyright by default, as soon as you fix the work in any medium (including digital). That is, if it’s in your head and you tell the story, no copyright. As soon as you write it on anything but sand or a dirty car, you get an immediate, implicit copyright.
The question is whether you bother to register the copyright or not. I suggest you peruse the information at www.copyright.gov. Briefly: you have to register before you can sue. If you register before the violation, you can claim more in damages. By default, you get “actual damages” which is the income you lost due to the violation. If it’s registered at the time of the violation, you can also get attorneys’ fees and “statutory damages”, which I believe is also known as punitive damages.
Now here’s the funny part. In the US, you’re required by law to file a copy with the Library of Congress when you publish, or face a fine. I wonder if this fine is ever levied. Last time I asked my father (a now-retired patent attorney), it hadn’t been adjudicated whether posting something on the web is considered “publishing”. Presumably posting on a blog or bulletin board isn’t, or the LoC would get flooded with more than it could possibly handle!
All the above is covered in www.copyright.gov. You can file a copyright without needing an attorney. My father used to walk clients through it the first time, training them to do it for themselves in the future. You generally need an attorney only if there are issues, questions, violations, accusations of violations, etc.
Back to the original question: lots of people post books for free. What is your goal: to make money, or for your book to be read? If the latter is more important than the former, go for it. I’m a musician; I post my music free for anyone to download. I’ve probably walked away from ohm maybe as much as $50 in lost revenue! No regrets. 
Definitely look into Creative Commons. This is a way to let everyone know, in very plain language, what they can and can’t do with your work, backed by legalese (which is almost never required, but nice to know it’s there.) Not only is it great that it makes it clear, but it gives you a few options you may not have considered. For example, would it be OK for someone to distribute your work without attributing it to you? (I would guess not.) The licence I use is “non-commercial, share-alike, with attribution”:
- It’s free to use for non-commercial purposes (if you want to sell it, please contact me first and we can work something out.)
- It’s OK to produce a derivative work, but your derivative work can’t have a license that’s more restrictive than mine (“share alike”).
- If you distribute it or include it in another work, you have to attribute it to me.
There’s a fixed set of reasonable options; all you have to do is pick the one you want, and then state it (or use the nifty little icon image) wherever you post your work. Pure gold!
I’m a big supporter of copyrights. I believe people deserve to get the benefit of their creations, if they so choose. The only benefit I want from my hobby creations is the fun of sharing them. Of course, I’d just love to get a call from a big-time producer for a well-known artist. I’d probably let the first one go royalty-free, just to get the grins of seeing my name on the credits! OK, I’m done dreaming here.
I’ve been coy about sharing my author info right on the message boards in the past, but, what the heck. Since you’re asking. 
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