Hi,
I've tried this basic project: http://arduino.cc/en/Tutorial/LiquidCrystal using the LCD included with the Starter Kit.
But when I download software to the Arduino I got this dark screen when I set the trimmer to half (see the photo in attachment), while if I turn to the maximum I got a dark yellow screen (and characters become invisible).
Could be a problem related to the LCD? Could be related to the wiring? :~
Thank you.
In your image, I don't see that you've connected the LED power to pins 15 and 16.
Yes, I didn't connect PIN 15 and 16 since in the schematics shown in the tutorial that I followed the Power seems to be supplied by PIN 2 (+5V) and PIN 1 and 5 (GRND). 
So do you think I have to disconnect PIN 1, 2 and 5, and connect PIN 15 and 16?
Thank you.
No, you need to connect 15/16 in addition to what the tutorial told you to connect. Not all LCDs have backlights and not all applications require them. So the tutorial treats them as optional.
If you've gotten "Hello World" to appear dimly, it shows that you've done everything right so far and that the LCD is working properly. Connect the backlight power and all will be good.
I just made a test! It works! Thank you! 
Good for you. And now back to Gothic Rock.
