You can read and edit pages without creating an account or logging in. To create an account (which is free), just click the "create an account or log in" link at the top right corner of any page. You only need a single login for all Wikia.
Creating an account is the only way to clearly attribute your work. Without a username, edits you make on any Wikia will be attributed to the numerical IP address of your computer instead. Logging in therefore also increases your privacy, because your IP address will be hidden once you are logged in.
There are many other benefits, such as user preferences, the ability to move pages, and a watchlist to keep an eye on selected pages. See "Why create an account?" on Wikipedia for some more arguments.