There is a search box at the top right hand corner of every page of the site. Enter one or more words to search for into the box and click the 'Go' button. The search will find pages containing as many of the words as possible. If you want to search for an exact phrase, place the phrase inside speech marks, e.g. "registering to vote". Alternatively, you can use the form on this page which is larger.
If you are having problems locating a page or document, you can read the Search Tips or try the Advanced Search.