I've been playing piano by ear for a long time and now I am trying to learn the first movement of the Moonlight Sonata. I am a little confused about the meaning of a little cross placed right before a note ; if I rely on my ear it seems to cancel a sharp or something, but the meaning is not quite clear! If you understand the symbol I'm describing help me out here!
I'm assuming that you're talking about the one that looks like a blocky X.....this is a double sharp. Instead of shifting the tone up one half step, it shifts the tone up 2 half steps (i.e. 1 whole step).
This image shows G double-sharp in the treble clef, and E double-flat in the bass clef. G double sharp is enharmonic with A natural, and E double-flat is enharmonic with D natural.
Coincidentally, Moonlight Sonata is the piece where I first encountered double-sharps, too!