Particle Plane Collision

Am writing a function to detect particle collision on a given plane, with the help of the normal of each point on the plane. I wanted to know how i can calculate the normal for each point on the plane.

My plane rendering code is,

/* Draw the floor. */
glColor3f(0.3, 0.3, 0.3);
glVertex3f(-12, -6.5, -12);
glVertex3f(12, -6.5, -12);
glVertex3f(12, -6.5, 12);
glVertex3f(-12, -6.5, 12);

Am really stuck, i would really appreciate the help.
Thank you

Please don’t cross post to multiple sections of the forum.

I’ve tried to answer your post in the advanced section, because I missed the duplicate post here. It should probably only been posted in this section.