How do you add a counter or something that show how many seconds,minutes and hours you have played?

Go into the tutorial section, and look at Pugs guide on paint.

Declare these as globals (secs,mins,hrs can be declared inside onPaint() if you'd like)

long startTime;
long timeRunning;
long seconds, minutes, hours;

and set

onStart(){
   startTime = System.currentTimeMillis();
}

and

onPaint(Graphics2D g){
   timeRunning = System.currentTimeMillis() - startTime;
  
   hours = timeRunning / 3600000;
   timeRunning -= hours * 3600000;
   minutes = timeRunning / 60000;
   timeRunning -= minutes * 60000;
   seconds = timeRunning / 1000;

   // and now just print it, for example
   g.drawString("Time: " + hours + "h " + minutes + "m " + seconds + "s", x, y);
}