Hello and welcome to Wikipedia. As of December 2001, we have created over 18,000 encyclopedia entries. We started in January, 2001, so we've made incredible progress--even we are surprised. The next really big milestone is 100,000 articles. At the rate we're growing, we might achieve this within four years!
Ordinary smart people, like you, are building this project very actively; as you are reading this, someone is probably editing an article. You don't even have to be logged in to contribute, but we'd like to know who you are. Anyway, you can just dive right in and work on any page you like. Please drop in as often as you can. If you're the kind of person that we are, you'll soon be addicted to the Recent Changes page. You can (and should) be bold in updating pages. And above all, play nice.
One of the most important things you should know is that we (the existing community) have established some cultural norms that you should respect. We try not to argue. This isn't a discussion forum. Everyone tries to reach a consensus about what entries should say. We try to make the entries as unbiased as we can, which means that the articles, even on controversial topics, are not meant to be platforms for preaching of any kind. Bear in mind that this is an encyclopedia, which means there are many things that it is not. If you feel uncomfortable with making changes to someone else's work, and you want to add your thoughts, questions or comments about an article, the place to do that is the article's /Talk page. Again, though, we generally try to avoid discussion for its own sake. There are also some other community norms codified via some links listed below, particularly Wikipedia policy. And, you might want to review the most common Wikipedia faux pas.
Humor in articles is fine and can make this project more fun and more readable. But this is intended to become an important basic resource on all areas of human knowledge, so please don't treat it as one big joke. Entries should be lively and readable, and they should also explain things as clearly as possible.
You might expect Wikipedia to be a low-quality product because it's open to everyone. But, perhaps it's the fact that it is open to everyone that makes a lot of these articles pretty good, and ever-improving. To alter a now-famous catchphrase: "Given enough eyeballs, all errors are shallow." We tend to cater to the highest common denominator--"lower denominators" tend politely not to touch articles they know nothing about! There are a lot of Ph.D.'s and graduate students and other very smart and knowledgeable people at work here--but everyone is welcome. We hope you have fun. If you're intrigued by the idea but you have objections, see our replies to common objections.
Still, if you think this is all too wild-and-woolly, then a more serious project will probably be better for you. Try Nupedia, which is much more rigorous and serious. (Many Wikipedians are Nupedians who are having fun with a neat side project that moves faster, but has fewer official quality controls--though one should not undervalue the fact that anyone can and often will immediately correct your mistakes.)
There is more introductory information: