To create a wiki, go to your Moodle course, turn editing on, click on Add an activity and select Wiki.
Please use the following resources on the http://docs.moodle.org/en/wiki to learn more about wikis in Moodle:
a) There is a main help page (what is a wiki, links to how to edit a page).
b) Finally, there is an entire chapter from a book called Using Moodle that is dedicated to
wikis. The PDF of the wiki chapter is available for free online.