ok, i did it. I forgot to use pushMatrix and pop Matrix
But i need help to make it smarter, for example i want to give some light effects (because i want to put noStroke to make it less "heavy"). i tried but i don't understand how to not influence also the background and the connecting green lines. And also it could be interesting to make sphere's collision (now the sphere enter inside the other).
Any help it will be very appreciated!  
(hey, sorry if my english is not really good, but i hope you can understand what i said )

/* Physic Bouncing Sphere */

import processing.opengl.*;


int numSpot = 4;
boolean oneTouched = false;
Spot[] sp;

void setup(){
 size(400,400,OPENGL);
 //smooth();

 sp = new Spot[numSpot];
 for(int i=0;i< numSpot;i++){
   sp[i] = new Spot();
 }
}

void draw(){
 background(14,36,48);

 stroke(100,200,134);
 strokeWeight(2);
 for(int i=0;i< numSpot;i++){
 for(int j=0;j< numSpot;j++){
   line(sp[i].x,sp[i].y,sp[j].x,sp[j].y);
 }
 }
 
 for(int i=0;i< numSpot;i++){
   sp[i].display();
   sp[i].move();
 }
 
 if(mousePressed==true){
   for(int i=0;i< numSpot;i++){
     if(sp[i].touched){
       sp[i].x = mouseX;
       sp[i].y = mouseY;
     }
   }
 }
}

void mousePressed(){

 for(int i=0;i< numSpot;i++){
   float d = dist(mouseX,mouseY,sp[i].x,sp[i].y);
   if(d < sp[i].r/2 && oneTouched==false){
     sp[i].touched = true;
     oneTouched = true;
   }
 }
 if(oneTouched == false){
   for(int i=0;i< numSpot;i++){
     sp[i].x = mouseX;
     sp[i].y = mouseY;
     sp[i].speedY = random(-1,1);
     sp[i].speedX = random(-2,2);
   }
 }
}
void mouseReleased(){
 for(int i=0;i< numSpot;i++){
   if(sp[i].touched){
     sp[i].speedY = random(-1,1);
     sp[i].speedX = random(-2,2);
   }
   sp[i].touched = false;

 }
 oneTouched = false;
}



/***************************************************************
                            CLASS
***************************************************************/


class Spot{

 // --> VARS
 float x,y;
 float r;
 color fillColor;
 float speedX, speedY;
 // acceleration force
 float gravity;
 // stop motion
 float damping, friction;
 boolean touched = false;


 // --> constructor
 Spot(){
   x = width/2;
   y = height/2;
  r = 30;
   
   speedY = random(-1,1);
   speedX = random(-2,2);
   gravity = 0.5;
   damping = 0.8;
   friction = 0.9;

   fillColor = color(252,58,81);

 }

 // --> METHODS
 void display(){
fill(fillColor);
   
   //noStroke();
   
   pushMatrix ();
   translate (x,y);
 
   sphere (r);
   popMatrix();
 }

 void move(){
   x+=speedX;
   speedY+=gravity;
   y+=speedY;
   
   // Check display window edge collisions
 if (x > width-r/2){
   x = width-r/2;
   speedX*=-1;
 }
 else if (x < r/2){
   x = r/2;
   speedX*=-1;
 }
 else if (y > height-r/2){
   y = height-r/2;
   speedY*=-1;
   speedY*=damping;
   speedX*=friction;
 }
 else if (y < r/2){
   y  = 0;
   speedY *=-1;
 }
 }
}

sorry, me too i'm not good with 3D (by the way, i'm not good at all  ). Can you give me a little suggest how to check the distance?

yeah, I know it's a forced 3D . But it's an experiment, I will improve better in future.
Thanks for your suggest, i'm understanding it's more complicated than I was thinking  .
By the way the two links about learning are very useful! I will read and I will try to solve the problem. I will post the result!