Let’s start simple. The following listing creates a script that draws a handful of randomly placed circles every time the mouse is clicked.

int _num = 10;
void setup() {
  size(500, 300);
  background(255);
  smooth();
  strokeWeight(1);
  fill(150, 50);
  drawCircles();
}
void draw() {
}
void mouseReleased() {
  drawCircles();
}
void drawCircles() {
  for (int i=0; i<_num; i++) {
    float x = random(width);
    float y = random(height);
    float radius = random(100) + 10;
    noStroke();
    ellipse(x, y, radius*2, radius*2);
    stroke(0, 150);
    ellipse(x, y, 10, 10);
  }
}