How do I castle?

When I am playing a game online, how do I castle? I have tried just moving the pieces to where I want them to be, but that does not work. I am sure I will kick myself but how do you do it?

You move the king 2 squares and the rook automatically swings around

http://www.conservativebookstore.com/chess/castling.htm

visit this link.its really helpful.

Pick up the king and move it two squares towards the relevent rook. The move will be completed automatically. Be careful that you don't castle whilst in check, through check or into check. If the King or rook have previously moved you will also be unable to castle.

one more tip-->if the space between the rook and the king is threaten by a figure of your opponent,you cant do it.

http://chess-video.blogspot.com/2009/12/chess-castling.html

see this short video on castling.