At the time of writing, the English version of Wikipedia has almost 4.5 million articles, and there are 286 other editions of Wikipedia for different languages.
Let me show you how to start editing! Go to Wikipedia and use the search box at the top right to find an article on something that you're interested in. Alternatively, you can click on the Random article link on the menu on the left, and you'll be taken to some weird and wonderful article. Have a look and see if there's something that you could improve; maybe by changing some wording to make it clearer, fixing spellings or punctuation, adding more information, or removing something unnecessary.
Now click on Edit (either at the top of the page, or at the top of a section) and you'll be taken to a page where you can add, change and delete the article's content. This uses a form of HTML coding called Wiki Markup, which isn't actually as complicated as it may first seem. Make your change (just type some random text if you want) and add a short explanation of what you've done in the Edit summary box. (Note that some articles, usually those for high-profile or politically contentious subjects, are protected to reduce vandalism and edit-warring – this is indicated by a padlock symbol at the top right. As a new user, you won't be able to edit these articles.)
Now click on the Show preview button at the bottom, and you'll see how the page would look with your changes. If you're happy with what you've done, then click Save page (don't do this unless it's a real edit though) and it'll be published. Well done, you've made your first Wikipedia edit!
Don't worry about making a mess. Every change made to an article is recorded in its revision history. This allows you to review previous versions of the article, and you can easily revert back to earlier content. You can't break Wikipedia! (If you do make a mess, please do revert back to the previous text; don't rely on someone else to tidy up after you.)
I won't go into too much detail about the Wiki Markup language here – you should pick it up as you go along by looking at how existing articles are structured – but I will explain how to create links to other articles. In each Wikipedia article, you'll see some words written in blue text. If you click on these you'll be taken to the linked article. (Sometimes you will also see red text – these are links to articles that don't exist yet.) To add a link, you simply put double square brackets around the words in the Markup, e.g. [[Geocaching]] will create a blue link to the Geocaching article. If you want to display different text to what you're linking to, then use the pipe character, e.g. [[Geocaching|my hobby]] will display "my hobby" in blue, but the link will go to Geocaching.
Articles also have corresponding discussion pages. Click on the Talk link at the top of the page. Sometimes these are more interesting than the actual articles! These can be edited in exactly the same way as the article pages, but there are a few rules to remember: new text goes at the bottom of the page, you should indent paragraphs to make the discussions easier to read (colons are used to shift paragraphs right, e.g. ::: will indent it three times), and you must remember to sign your posts. You can do this by clicking on the signature button above the edit area, or by typing ~~~~ after your text.
You don't need to register on Wikipedia to be able to edit articles (apart from some of the protected ones). If you're not logged in then your edits will be recorded against your computer's IP address. However, registration will give you certain benefits, such as your own User page and corresponding User Talk page, and the ability to set up a Watchlist of articles that you're interested in, enabling you to easily monitor changes to these made by other editors.
Oh, and the cache, I almost forgot. It's a plastic box hidden somewhere in Latchford, not at the published coordinates. Good luck with finding it, and with your Wikipedia editing!