Sheet music - Cross sign

[rainyday]

Hey pianostreetforumguys.  

I'm a self taught amateur pianist (been playing +- 2 years now) and I'm trying to learn Moonlight Sonata 1st mvt. It's going quite all right but I ran into a "cross" sign which I haven't seen before. Google and searching this forum didn't get me any results so far.

So I'm requesting for a little bit of help here. I'm kinda stuck because I don't want to learn it in a wrong way so I have to waste time correcting it.

Many thanks in advance~

[birba]

Double sharp.  two halfsteps up.  e.g. f double sharp would just be a plain g.  It doesn't matter if there's already a sharp sign for that note in the key signature.

[rainyday]

Ok I got that, thank you very much Birba.