The policies of every wiki are a little bit different, so you will need to visit the site you wish to contribute to and research the specific details that apply to that wiki. Some wikis require that you create an account, while others will allow you contribute without logging in. The thing to keep in mind is that wikis are edited by the same users that contribute to the site, as a result you can expect any contribution you make to be seriously discussed and edited by other contributors. As a result, at no point will you be able to point to a page on any wiki and claim it as your own work. Every wiki entry is a collaborative product, and no one person can claim credit for the contents of a wiki.