The straighforward way to do it is to have a global pointer to the top of the stack:

#include <stdio.h>
#include <stdlib.h>
#include <assert.h>

struct node {
  struct node *next;
  int data;
} *top; // the scope of *top is now global

typedef struct node node;

void push(int value)
{
  node *new = malloc(sizeof(node));
  assert(new != NULL);
  new->data = value;

  new->next = top;
  top = new;
}

void pop()
{
  node *temp = top;
  top = top->next;
  free(temp);
}

void display()
{
  node *sp = top;
  while (sp != NULL)
    {
      printf("%d ", sp->data);
      sp = sp->next;
    }
  printf("(over)\n\n");
}

int main() {
  top = NULL;
  push(10);
  push(20);
  pop();
  push(30);
  display();
  pop();
  display();
  push(99);
  display();

  return 0;
}

However, it is also possible to get rid of the global variable by passing around the address of the stack pointer:

#include <stdio.h>
#include <stdlib.h>
#include <assert.h>

struct node {
  int value;
  struct node *next;
};

typedef struct node node;

void display(node  *n)
{
  while (n->next != NULL) {
    printf("%d --> ", n->value);
    n = n->next;
  }
  printf("%d (end).\n\n", n->value);
}

void push(node **sp, int v)
{
  node *newNode = malloc(sizeof(node));
  assert(newNode != NULL);
  newNode->value = v;

  newNode->next = *sp;
  *sp = newNode;
}

void pop(node **sp)
{
  node *temp = *sp;
  *sp = (*sp)->next;
  free(temp);

  printf("pop!\n\n");
}

int main() {

  node *stackPointer = NULL;

  push(&stackPointer, 10);
  push(&stackPointer, 20);
  push(&stackPointer, 30);
  display(stackPointer);
  pop(&stackPointer);
  display(stackPointer);
  push(&stackPointer, 99);
  display(stackPointer);

  return 0;
}

A nice diagram showing the push() function:

The Python version is pretty much the first C version written in OOP style.

#!/usr/bin/env python

class Node:
    def __init__(self, value):
        self.value = value
        self.next = None

class Stack:
    def __init__(self):
        self.stack = None

    def push(self, value):
        new = Node(value)
        new.next = self.stack
        self.stack = new

    def pop(self):
        self.stack = self.stack.next
        print 'pop!\n'

    def display(self):
        i = self.stack
        while i != None:
            print i.value
            i = i.next
        print '(end)\n'

def main():
    s = Stack()
    s.push(10)
    s.push(20)
    s.push(30)
    s.push(40)
    s.push(50)
    s.display()
    s.pop()
    s.pop()
    s.display()
    s.push(99)
    s.display()

if __name__ == '__main__':
    main()