Code

 Header...
#define NDIM 2
typedef struct particle {
  float x[NDIM];
  float mass;
  struct particle *next;
}Particle;

// Allocate a new particle and add it to the start of the list
 struct particle* newparticle(Particle *first){
   ... 

»  Particle *p;

»  p=xmalloc(sizeof *p);
»  p->next=first;

  // Now initialise exactly as before
»  printf("Please enter the mass and x and y values.\n");
»  scanf("%f %f %f", &p->mass, &p->x[0], &p->x[1]);

»  return p;
}

 int main(){
   ... 

»  int i, n;
»  Particle *first=NULL;

»  printf("How many structures would you like?\n");
»  scanf("%d", &n);

»  for (i=0; i<n; ++i) 
    first=newparticle(first);

»  return 0;
}