Like any website too large for a simple directory to make sense, Wikipedia's on-site search function is an absolute necessity. This box appears in the top right of all pages, and it works like any other: type your query in the box and hit "enter" to visit the search results page.
The search feature offers spelling and query corrections, and hyphens can be used to turn "yes include" queries into "no" terms. Keep in mind, characters other than letters and numbers are ignored. Eventually we'll go more in-depth, but for now, happy searching!