Well, you know the value of PI, all you have to do is create either a variable or a macro for it.
#define PI 3.14159265359f
// float PI = 3.14159265359f; // This would work just as fine
float y = PI * 4.0f;
printf("y = %f\n", y);
You can also use it directly by adding another library in your code.
The library is math.h, I will demonstrate it by calculating area of a circle, so it would look like,
printf("Enter the value of radius of circle\n");
printf("The area of the circle is %f\n",M_PI*r*r);
Hope it helps