How do I castle?

Regular Chess:
Move your king two squares to the right or the left and the rook will move itself to the proper square.
Castling can only be made when:
- There are no pieces in between the Rook and the King.
- The King has not yet moved.
- The Rook that is castling with the King has never moved.
- The King is not in check
- The King will NOT be crossing squares that are attacked by opponent's pieces.
For pictures CLICK HERE. 

Chess960:
Castling is basically the same as in regular chess, except the king and rook may start on different squares from regular chess. If your king is moving fewer than two squares when it castles, you can make the move by typing "OO" (or "oo" or "O-O") for kingside castling or "OOO" (or "ooo" or"O-O-O") for queenside castling. If you are using BlitzIn 2.6+ or Dasher 1.1.2+ you can also castle by dragging the king on top of the rook you are castling with. These methods work for any castling situation. If your king is moving two squares or more, you can just move the king and it will be understood that you intend to castle.  
For more information read "help Chess960"