Castling Issues

I'm new to this so please forgive my ignorance but how do I castle in a game? It wont let me swap the king and the rook over even though they are in a castling position. What do I click on  to make it happen please???

Scroll down to find "castling"

http://www.chess.com/learn-how-to-play-chess

Thanks for the reply. I know what castling is I am just unable to do it in the game. Do I move the rook first? the king first? How do I swap them over?? It wont let me. 

Click the king and move it two squares in the direction you want to castle

The King move first, make sure is doesn't come into check or come across a path that will come under attack.

You didn't read it very well did you.

Castling

One other special rule is called castling. This move allows you to do two important things all in one move: get your king to safety (hopefully), and get your rook out of the corner and into the game. On a player’s turn he may move his king two squares over to one side and then move the rook from that side’s corner to right next to the king on the opposite side. (See the example below.) However, in order to castle, the following conditions must be met:

• it must be that king’s very first move
• it must be that rook’s very first move
• there cannot be any pieces between the king and rook to move
• the king may not be in check or pass through check

Thanks. Much appreciated.