String myText = "Type something";
 
void setup() {
  size(500, 500);
  textAlign(CENTER, CENTER);
  textSize(30);
  fill(0);
}
 
void draw() {
  background(255);
  text(myText, 0, 0, width, height);
}
 
void keyPressed() {
  if (keyCode == BACKSPACE) {
    if (myText.length() > 0) {
      myText = myText.substring(0, myText.length()-1);
    }
  } else if (keyCode == DELETE) {
    myText = "";
  } else if (keyCode != SHIFT && keyCode != CONTROL && keyCode != ALT) {
    myText += key;
  }
}