Frankieve, keep in mind the key you are playing in. Say the key is "C," some substitutions for C are Am, C6, Cmaj7, even an occasional Ab. Substitutions for the "F" chord might be a Dm, Dm7 or Gsus, etc. In addition to the key signature, follow the melody and see which chord in that key signature fits best. As you get better, you will find even more substitutions.