At Admin Panel -> Settings -> Switches you have an option to activate the search log. Once activated, all searches performed by your visitors will be recorded. Once the switch is on and some searches have happened, go to Admin Panel -> Miscellaneous -> Search Log. This page will show you a list of the most frequently searched terms, as well as allowing you to find how many searches there've been for any particular term. You can use this data to consider what new content you'll want to add (or how you may want to re-title existing content) to fill in the gaps in what people are looking for.

If you want to display the search data in some other way or somewhere else you'll need to create a toplist -- go to Admin Panel -> Themes -> Toplist Generator and select to do a toplist of searches. See Search Log Variables for an explanation of the template variables you'll be able to use in those toplists.