#include <stdio.h>
void triangle(int N)
{
  for(int i=0;i<N;i++)
  {
    putchar('\n');
    for(j=0;j<i;j++) putchar('*');
  }
}
 
int main()
{
  triangle(5);
}

That should do it. I'm sure you can figure out how to get user input for the numer N. Try scanf!