rgPaiva:
I already did it, but it only shows 1023...
So you are saying that even running a wire from A0 to ground you are getting 1023? Are you connecting a wire from A0 to the GND port on the Mega directly?
If so something is wrong. Even if there is a short running it directly to ground should bring it down.
Is your Mega sitting on top of a metal table or something? There are no shields installed right?