1. First, you need a search engine. Google will be fine. XD
2. Add this word to the search engine --> site:mediafire.com
3. then add the name of the file that you want. XD
4. If you want more specific search you can try this
to search all music files hosted on MediaFire type this in Google:
Or for a specific artist, album or song include the artist, album or song name:
site:mediafire.com mp3|wma|ogg "type your artist, album or song name here"
For video files, use this:
site:mediafire.com asf|rm|avi|mp4|wmv|flv "type your artist, album or video name here"
For Comics, use this:
site:mediafire.com cbr|cbz "Comic name here"
For compressed file-archives and software:
site:mediafire.com rar|zip|exe "Search Query Here"