If you really want to understand what is going on, I suggest reading this book:
and the cases and statutes it references. But this is really unnecessary to get something decent.
I don't recommend hiring a lawyer. They will either go through basically the same process as described above anyway and then charge you a lot for it. If you have a specific issue with your site, you might want to hire a lawyer to help you draft specific provisions that deal with that issue. But seriously, do you have an issue that facebook/craigslist/ebay/microsoft/amazon/etc. don't?
Fresh book recommendations delivered straight to your inbox every Thursday.