First off, welcome! Post up some pictures of your Mustang!
Piggybacking on what the other two members have said, start with checking the clutch quadrant (located under the dash) a good inspection. If you decide to replace the clutch cable i highly recommend getting either a Ford, Steeda or Maximum motorsports cable. Almost all of the other cables are not made to the same standard and often snap, leaving you without a clutch and making a drive home much more difficult.
If the clutch still hasn't improved, then consider replacing the clutch with anything the local parts store has or even something off of RockAuto.