Searching the AoPSWiki is rather easy. You type in your search query in the box on the right side of the screen and click the Search button. Make sure not to make any spelling errors in your query, otherwise you may get unintended results.
Below the search textbox there are two buttons: Go and Search. If you click Go, then you will go straight to that page. Therefore, you must have it typed it in perfectly to get there. One thing to note is that the AoPSWiki article names are case sensitive (except for the first letter of every article, which is case insensitive).
The AoPSWiki search function does have mercy on you, though, if you meant to hit Search instead of Go. Assuming you did not actually type in the name of a real article, you will see the search results for the phrase you typed in.
If, instead of searching for certain things, you'd rather browse through the wiki, the Table of Contents is an excellent place to start.