Sorting numbers in a file....How do you sort in a file?

#include <stdio.h>
#include <stdlib.h>
 
int main()
{
   char ch, file_name[25];
   FILE *fp;
 
   printf("Enter the name of file \n");
   gets(file_name);
 
   fp = fopen(file_name,"r"); // read mode
 
   if( fp == NULL )
   {
      perror("Error while opening the file.\n");
      exit(EXIT_FAILURE);
   }
 
   printf("The contents of %s file are :\n", file_name);
 
   while( ( ch = fgetc(fp) ) != EOF )
        
      printf("%c",ch); 
   
   fclose(fp);
   
   
   return 0;
}