Hello. How can I write assert function for my code?

how can i write assert function to my code? Thanks.

#include <stdio.h> 
#include <string.h> 
#include <assert.h> 
int main ()
{
   char string[99]; 
   int count; 
   {
   printf("\tThe First Part.\nEnter any string: ");
   scanf("%s",string);
   for(count=0;string[count]!='\0';++count); 
   printf("Length is: %i",count); 
   }