Processing Mouse and Keyboard Input

First, know the processing automatically records your last mouse button in a special variable called mouseButton as well as the last key you pressed as keyCode.

These values are recorded as numbers. Example:
mouseButton values:
Left click == 37
Right click == 39

keyCode values
a-key == 65
b-key == 66
...
z-key == 90

Left-arrow key == LEFT
Right-arrow key == RIGHT

Each time you press a key, processing automatically calls a function named keyPressed.
Each time you release a key, processing automatically calls a function named keyReleased.

Example:

//This script will draw a circle on the screen if the a-key is being pressed
//and it will draw a rectangle on the screen if the b-key is being pressed
//it will not draw either shape if these keys are not being pressed

var a_pressed = false;
var b_pressed = false;

var draw = function() {
  background(200)
  if (a_pressed) {
    ellipse(250,250,100,100);
  }
  if (b_pressed) {
    rect(150,150,200,200);
  }
}

var keyPressed = function() {
  if (keyCode == 65) {
    a_pressed = true;
  }

  if (keyCode == 66) {
    b_pressed = true;
  }
}

var keyReleased = function() {
  if (keyCode == 65) {
    a_pressed = false;
  }

  if (keyCode == 66) {
    b_pressed = false;
  }
}