Once you've downloaded & installed LW & it's got past all of the 'loading' messages, the page which comes up is where you start your search. On the left side of the page you'll see 'Select Search Type' & then categories (All Types is default)...choose your category & then type the file name that you're searching for into the text field & click on 'search' Then a page will show with the search results.

Check out the links below for more info.
User Guide - LimeWire

How to find music (tips for video also)