Legal move

What do I do when I can't make a legal move?

The game is over, and so on this site, you shouldn't do anything. 

You will never find yourself in this situation.

If you have no legal move, the game will have already ended through either a win by  checkmate for your opponent, or a draw through stalemate.

That what I was looking for. who determines the stalemate?

Thanks for the reply.

Stalemate is recognizable from the board position. Why would it need to be "determined" ?

The system automatically recognizes stalemate and will immediately end the game. If you get a position you think should be stalemate and the game hasn't ended, then check more deeply because there is still a legal move for you on the board. Don't forget about en passant.

Now that's the answer I was looking for thanks a lots, Martin_Stahl.