1. // Global var:
2. boolean bShowStartScreen = true;
3.  
4. // In draw:
5. if (bShowStartScreen)
6. {
7.   // Display the start screen
8.   return;
9. }
10. // else
11. // Display your game
12.  
13. void keyPressed()
14. {
15.   if (key == ' ') bShowStartScreen = false;
16. }
17. 
    

Thank you very much for the patience.
Sorry but I haven't understand how to write the program properly with your code, because I started used Processing only a month ago..

In the global var (after PFont font;), I wrote boolean bShowStartScreen = true;

But what I have to write in the void draw?

And where I have to put the void keypressed()?

What PhiLho gave you was just a blueprint model/template what you still have to code!
You already have lotsa code. You only have to mold them after the given template.

@ #5 -> if (bShowStartScreen) { ... }

you put your "Start Screen" drawing within that if block.

And keyPressed() is another "callback" function, just like setup() & draw().
You can place it anywhere outside any other functions.

EDIT: Wait, I've done it, thanks :)
Now, as I asked in the first post, I wanted to produce an alert sound in the background during the "start" screen.
Could you suggest me how to do this?

http://processing.org/reference/libraries/
Look at the Minim library: it comes with Processing and has several examples showing how to use it.

Thanks.
I found this code in that page under Quickstart guide, but it doesn't seem to work.
It gives to me an unexpected char: "\" error..

1. [snip java]
   import ddf.minim.*;

   Minim minim;
   AudioPlayer song;

   void setup()
   {
   size(100, 100);

   minim = new Minim(this);

   // this loads mysong.wav from the data folder
   song = minim.loadFile(“mysong.wav”);
   song.play();
   }

   void draw()
   {
   background(0);
   }

   void stop()
   {
   song.close();
   minim.stop();

   super.stop();
   }
   [/snip]

Yes, it trips lot of people, as this site seems to "auto-correct" regular straight double quotes into curvy ones that are not correct code:
song = minim.loadFile( “mysong.wav ”);

Perfect, now it works everything :)
Only one final problem.
If I press Esc, when playing the game freezes and in the code is highlined the string "song.close();" and I have to quit the game by pressing the "stop" button in the code.

The error message is this:
Exception in thread "Animation Thread" java.lang.NullPointerException
    at ddf.minim.javasound.JSBaseAudioRecordingStream.close(Unknown Source)
    at ddf.minim.AudioPlayer.close(Unknown Source)
    at aviraproj.keyPressed(aviraproj.java:53)
    at processing.core.PApplet.handleKeyEvent(PApplet.java:2931)
    at processing.core.PApplet.dequeueEvents(PApplet.java:2466)
    at processing.core.PApplet.handleDraw(PApplet.java:2153)
    at processing.core.PGraphicsJava2D.requestDraw(PGraphicsJava2D.java:193)
    at processing.core.PApplet.run(PApplet.java:2020)
    at java.lang.Thread.run(Thread.java:662)

Okay, I solved this too :)
I had to cancel the strings

1. song.close();
   minim.stop();

from my code. I don't know why, but it worked. Lol.

" to cancel the strings"
These are not strings...

" I don't know why"
At least to avoid the error you got!