The same rules apply in the cyberworld as in the print world.
Web sites, too, can be evaluated on the basis of authority, objectivity, accuracy, currency, content/relevancy, and aesthetics.
Anyone these days can put up a page on the World Wide Web. Teachers (and/or parents) have put up young school children's pages on various topics (but you don't know whether Johnny or Susie got an "A" on the assignment or not!).
You can also go with sources you know to be reliable in the print world, such as the Encyclopedia Britannica, the New York Times, or the U.S. government.
It used to be that I depended on checking for .edu (schools) or .org (nonprofits) or .gov in the Web address (URL) to help decide how trustworthy I thought the site might be.
But now I am seeing more sites that have .com or .net but yet are still from the U.S. (or Harris County) government. See Harris County Clerk and Harris County District Clerk, for example.
Generally speaking, .net and .com denote "network" and commercial sites, respectively (with exceptions such as noted above).
However, it's about to get much more confusing, as it appears that new generic "top level domain" names are now making their presence known on the Web such as .biz, .info,, etc.
And then there are addresses like the one for Harris County Public Library.
Another popular type of Web address that is easy to understand is the one for Aldine Independent School District.
One more thing. You may also want to watch out for country codes.
If you see an address like this, http://www.ox.ac.uk/, you can guess that it is for something in the United Kingdom and that it is an academic site. This is, in fact, the home page for University of Oxford. (See also the University of Tokyo's home page.) Outside the U.S., ac is the same as edu in U.S. school addresses.
Enough about domain names for now.
After you've decided that this or that site is a good source to cite for your paper, you probably will next need either the MLA Format site or the APA Format site to figure out what form your citation will take.
Lastly, common sense still applies. If you are not sure whether a site is worthy of your trust, it's probably not!