You put any variables back into the state they were in at the beginning of the sketch and the sketch will appear to start again.  If you're doing your drawing by overlaying new stuff then you can call background() to clear the window.  There's no single command that will restart the sketch.

I have this code:

int numBolas = 10000;
int tamanhoBolas = 10;
int velocidadeBolas = 4;

float[]largura = new float[numBolas];
float[]altura = new float[numBolas];
float[]xposicao = new float[numBolas];
float[]yposicao = new float[numBolas];
float[]xvelocidade = new float[numBolas];
float[]yvelocidade = new float[numBolas];

in[]colors = new int[numBolas];

void setup() {

  size(800, 600);
  background(0);

  for (int i = 0; i < numBolas; i++) {

    xvelocidade[i] = random(1, velocidadeBolas);
    yvelocidade[i] = random(-velocidadeBolas, velocidadeBolas);
    largura[i] = random(1, tamanhoBolas);
    altura[i] = largura[i];
    xposicao[i] = width/2+random(-width/20, width/20);
    yposicao[i] = height+random(-height/20, height/20);
    colors[i] = color(random(255), random(255), random(255));
    fill(colors[i]);
  }

  noStroke();
  frameRate(100);
}

void draw() {

  background(color(random(255), random(255), random(255)));

  for (int i = 0; i < numBolas; i++) {

    fill(colors[i]);
    ellipse(xposicao[i], yposicao[i], largura[i], altura[i]);
    xposicao[i]+=sin(i)*xvelocidade[i];
    yposicao[i]+=cos(i)*yvelocidade[i];

    if (xposicao[i]+largura[i]/2>=width || xposicao[i]<=largura[i]/2) {
      xvelocidade[i]*=-1;
    }
    if (yposicao[i]+altura[i]/2>=height || yposicao[i]<=altura[i]/2) {
      yvelocidade[i]*=-1;
    }
  }
}

my goal is to put the balls starting to explode every time I click and where I click.
How can I do it?

You have a bunch of code in your setup() function that initialises your sketch.  All you need to do is move all that do a separate function and then call the new function once in setup to get things going and then from a new mouseClicked() function that you need to add.

1. int numBolas = 10000;
2. int tamanhoBolas = 10;
3. int velocidadeBolas = 4;
4. 
   
5. float[] largura = new float[numBolas];
6. float[] altura = new float[numBolas];
7. float[] xposicao = new float[numBolas];
8. float[] yposicao = new float[numBolas];
9. float[] xvelocidade = new float[numBolas];
10. float[] yvelocidade = new float[numBolas];
11. 
    
12. int[] colors = new int[numBolas];
13. 
    
14. void setup() {
15.   size(800, 600);
16.   initialiseSketch();
17.   noStroke();
18.   frameRate(100);
19. }
20. 
    
21. void initialiseSketch() {
22.   for (int i = 0; i < numBolas; i++) {
23.     xvelocidade[i] = random(1, velocidadeBolas);
24.     yvelocidade[i] = random(-velocidadeBolas, velocidadeBolas);
25.     largura[i] = random(1, tamanhoBolas);
26.     altura[i] = largura[i];
27.     xposicao[i] = width/2+random(-width/20, width/20);
28.     yposicao[i] = height+random(-height/20, height/20);
29.     colors[i] = color(random(255), random(255), random(255));
30.   }
31. }
32. 
    
33. void draw() {
34.   background(color(random(255), random(255), random(255)));
35.   for (int i = 0; i < numBolas; i++) {
36.     fill(colors[i]);
37.     ellipse(xposicao[i], yposicao[i], largura[i], altura[i]);
38.     xposicao[i]+=sin(i)*xvelocidade[i];
39.     yposicao[i]+=cos(i)*yvelocidade[i];
40.     if (xposicao[i]+largura[i]/2>=width || xposicao[i]<=largura[i]/2) {
41.       xvelocidade[i]*=-1;
42.     }
43.     if (yposicao[i]+altura[i]/2>=height || yposicao[i]<=altura[i]/2) {
44.       yvelocidade[i]*=-1;
45.     }
46.   }
47. }
48. 
    
49. void mouseClicked() {
50.   initialiseSketch();
51. }

In the initialiseSketch() function change the lines where you set xposicao[i] and yposicao[i] so that those are equal to mouseX and mouseY instead of width/2 and height.  Add some random() to each of those if you want.

See http://openprocessing.org/collection/1496

BTW. I am not sure whether you want a new ball to start or kill an existing ball by exploding them. The collection I posted the link to is the latter (see lemmings e.g.), for the former, see http://processing.org/learning/topics/arraylistclass.html