Well, for starters, you'd have to be board certified or board eligible in anatomic and/or clinical pathology.
You DO NOT automatically get what you want, but competition varies greatly among both subspecs and institutions, from fairly easy to insanely competitive.
In addition, fellowship is increasingly seen as necessary for many positions, so applicant pool is likely to grow faster than # of fellowships.
Qualifications includes step scores, board scores, grades/schools, research etc. etc. Also, some programs want you to do two fellowships, e.g. surg path followed by pediatric path or whatever.
There's currently 10 subspecs which are certified by the American Board of Pathology:
Blood Banking/Transfusion Medicine
Chemical Pathology
Cytopathology
Dermatopathology
Forensic Pathology
Hematology
Medical Microbiology
Molecular Genetic Pathology
Neuropathology
Pediatric Pathology