I need to write a code to open a directory entered at the command prompt that finds all the files of one type(mp3) in the directory. What functions should I be looking at to do this?

There are no such functions in the standard library.
see boost : http://www.boost.org/doc/libs/1_31_0/libs/filesystem/doc/index.htm
or microsoft : http://msdn.microsoft.com/en-us/library/aa364418(VS.85).aspx