Wikipedia has become one of the most popular websites on the web not just because of its ease of use, but also because of the sheer vastness and timeliness of its information. It might take a regular encyclopedia a year or more to generate an entry for a new concept such as 'Web 2.0', but Wikipedia will have an entry as soon as someone takes the time to write one. And, with anything that catches the public's eye, that's usually pretty quick.
So how do you get in on the act?
Go to the Wikipedia website and sign into your account. If you don't have a Wikipedia account, it's very easy to get one. Just go to the main site and click on the Sign In / Register link.
Search for the article you want to write. You don't want to reinvent the wheel, so its best to check for any other articles on the subject. Use the search box that is on the middle left hand side of the main Wikipedia site.
If no articles came up, the page will tell you that no articles exist. Under this it invites you to "create this page." Click on that link and you are ready to enter your article.
You can also enter an article by modifying the URL. But this is more complicated and requires modifying the address bar of your web browser to point Wikipedia to an article that doesn't exist.
It's really much easier to just use the search box.