From the main Help page, you can use the search box on the right side of the page. Type in the terms you want to search for. By default, it will search for those terms only in the questions in the help. If you check the indicated box, it will also include answers that match your search terms. Also, words that are three letters or less are generally ignored by the search engine as being too common. Think of search terms that are four letters or longer. Only the first 100 matching results are listed.

Without any qualifiers, the search will assume you want to find entries that have any of your keywords in them. So if you search for National Park Service, it will return entries that have the words national, park, or service. An entry only needs to have any one of those three words to match. If you want to search for entries that have all three words, you should include the phrase in quotes (e.g. "National Park Service") which returns all posts with that entire phrase.

Additional qualifiers can help you narrow down your search further:

+	A leading plus sign requires that the word be in the search results.
-	A leading minus sign requires that the word not be in the search results.
( )	Parenthesis are used to group words into sub-groups
*	An asterisk added to the end of a word will match all words that start with the same word it has been added to
"	A phrase in double quotes will only match entries that have the entire phrase in it

Examples

apple banana	find entries that contain at least one of these words
+apple +juice	find entries that contain both of these words
+apple -macintosh	find entries that contain the word "apple" but not "macintosh"
+(apple banana) -macintosh	find entries that contain "apple" or "banana", but not "macintosh"
letterbox*	find entires that contain "letterbox", "letterboxer", "letterboxers", "letterboxing", etc.
"some words"	find entires that contain "some words of wisdom", but not "some noise words"