How can I change the metadata of my songs all at once?
You could also run a script like this in AppleScript Editor:

tell application "iTunes"
    repeat with t in selection
        tell t
            set name to my titlecase(name)
            set artist to my titlecase(artist)
            set album to my titlecase(album)
        end tell
    end repeat
end tell

on titlecase(x)
    do shell script "ruby -rubygems -KUe 'require \"titlecase\"
    print ARGV[0].titlecase' -- " &amp; quoted form of x
end titlecase


The titlecase gem can be installed with sudo gem install titlecase.