Calculate Normals in Shader - c++

Question

I am new to OpenGL and Shaders.
I want to write a toon shader.
I have this OpenGL code:
void display()
{
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glLoadIdentity();
float black[] = { 0.0, 0.0, 0.0, 1.0 };
float red[] = { 1.0, 0.0, 0.0, 1.0 };
float green[] = { 0.0, 1.0, 0.0, 1.0 };
float blue[] = { 0.0, 0.0, 1.0, 1.0 };
float white[] = { 1.0, 1.0, 1.0, 1.0 };
float lowAmbient[] = { 0.2, 0.2, 0.2, 1.0 };
float fullAmbient[] = { 1.0, 1.0, 1.0, 1.0 };
glMaterialfv(GL_FRONT, GL_AMBIENT, blue);
glMaterialfv(GL_FRONT, GL_DIFFUSE, blue);
glMaterialfv(GL_FRONT, GL_SPECULAR, white);
glMaterialf(GL_FRONT, GL_SHININESS, 128.0);
glLightfv(GL_LIGHT0, GL_AMBIENT, lowAmbient);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glTranslatef(0.0, 0.0, -9);
glRotatef(45, 1, 0, 0);
glRotatef(45, 0, 0, 1);
glBegin(GL_QUADS);
//front
glNormal3f(0.0, 0.0, -1.0);
glVertex3f(-1.0, 1.0, 1.0);
glVertex3f(-1.0, -1.0, 1.0);
glVertex3f(1.0, -1.0, 1.0);
glVertex3f(1.0, 1.0, 1.0);
//back
glNormal3f(0.0, 0.0, 1.0);
glVertex3f(1.0, 1.0, -1.0);
glVertex3f(1.0, -1.0, -1.0);
glVertex3f(-1.0, -1.0, -1.0);
glVertex3f(-1.0, 1.0, -1.0);
//right
glNormal3f(1.0, 0.0, 0.0);
glVertex3f(1.0, 1.0, 1.0);
glVertex3f(1.0, -1.0, 1.0);
glVertex3f(1.0, -1.0, -1.0);
glVertex3f(1.0, 1.0, -1.0);
//left
glNormal3f(-1.0, 0.0, 0.0);
glVertex3f(-1.0, 1.0, -1.0);
glVertex3f(-1.0, -1.0, -1.0);
glVertex3f(-1.0, -1.0, 1.0);
glVertex3f(-1.0, 1.0, 1.0);
//top
glNormal3f(0.0, 1.0, 0.0);
glVertex3f(-1.0, 1.0, -1.0);
glVertex3f(-1.0, 1.0, 1.0);
glVertex3f(1.0, 1.0, 1.0);
glVertex3f(1.0, 1.0, -1.0);
//bottom
glNormal3f(0.0, -1.0, 0.0);
glVertex3f(-1.0, -1.0, -1.0);
glVertex3f(-1.0, -1.0, 1.0);
glVertex3f(1.0, -1.0, 1.0);
glVertex3f(1.0, -1.0, -1.0);
glEnd();
//Swap back and front buffer
glutSwapBuffers();
}
void init()
{
glClearColor(0.0, 0.0, 0.0, 1.0);
glEnable(GL_DEPTH_TEST);
glLightModeli(GL_LIGHT_MODEL_LOCAL_VIEWER, GL_TRUE);
glEnable(GL_LIGHTING);
glEnable(GL_LIGHT0);
float ambientLight[] = { 0.2,0.2,0.2,1.0 };
float diffuseLight[] = { 0.8,0.8,0.8,1.0 };
float specularLight[] = { 1.0,1.0,1.0,1.0 };
float lightPosition[] = { 0.5,0.5,0.0,1.0 };
glLightfv(GL_LIGHT0, GL_AMBIENT, ambientLight);
glLightfv(GL_LIGHT0, GL_DIFFUSE, diffuseLight);
glLightfv(GL_LIGHT0, GL_SPECULAR, specularLight);
glLightfv(GL_LIGHT0, GL_POSITION, lightPosition);
}
My Shader Codes:
#shader vertex
#version 330 core
void main()
{
}
#shader fragment
#version 330 core
vec3 LightPosition = gl_LightSource[0].position;//I don't know this is true or not
vec3 Normal;//I don't know how to calculate
void main()
{
vec4 color1 = gl_FrontMaterial.diffuse + gl_FrontMaterial.specular +
gl_FrontMaterial.ambient;
vec4 color2;
float intensity = dot(LightPosition, Normal);
if (intensity > 0.95) color2 = vec4(1.0, 1.0, 1.0, 1.0);
else if (intensity > 0.75) color2 = vec4(0.8, 0.8, 0.8, 1.0);
else if (intensity > 0.50) color2 = vec4(0.6, 0.6, 0.6, 1.0);
else if (intensity > 0.25) color2 = vec4(0.4, 0.4, 0.4, 1.0);
else color2 = vec4(0.2, 0.2, 0.2, 1.0);
gl_FragColor = color1 * color2;
}
To calculate light intensity and apply colors to my cube object I should know normals.
How can I calculate, or if there is a way, reach them?
(I have no problem with the shader compilation, or other OpenGL stuff. If I close my shader compilation lines I can see a green cube.)