Now most chess players understand that it is important and a great idea to castle during the opening part of the game. A general rule of thumb is to castle within the first fifteen moves of the game. This is not always possible, but generally it is not too hard to accomplish. Castling protects your king during the middle game and simultaneously develops your rook. Knowing how devastating a rook can be to your opponent's defense makes castling an even better idea.
Try to develop your primary pieces to the side that you are going to castle to. Most of the time players will castle on the king's side of the board, not the queen's side. If this is the case then you want to develop your knight and your bishop to that side of the board. You also want to keep your primary pieces nearby, or close to the king so that they are not strung out across the board. Keeping your troops close to your king provides your king the protection that he needs while at the same time creating difficulty for your opponent to be able to advance.
When castling you also need to be sure to keep your front line of pawns in their place to provide that added barrier to help protect your king. These pawns also work as guards for your primary pieces that you have developed on the king's side. The pawns will help guard the bishop and knight that you have placed on that king side after castling. Think of your king as being the most valuable and weakest piece on the board. Knowing this, use all of your other pieces to work as protectors of that weak but valuable king that your opponent is so desperately seeking to capture.