The rook is a valuable piece, but its value usually doesn't show until later in the game. The more open the board becomes, the more maneuverable (and powerful) your rooks get. Until the board opens up, do not waste time moving your rooks around unless it is necessary. The best time to move a rook is when you are doing so to capture an open file. (Files are the columns of squares running from your end of the board to the opponent's end) When there is an open lane all the way from your end of the board to your opponent's, it is usually a good idea to control that lane by moving your rook there first. This way, he has to have something protecting his rook in order to move it onto the same file.
Another good placement of the rook is on a file where you do not have a pawn, but your opponent does. This is called a half-open file. Putting the rook there pressures that pawn, and if that pawn is captured or leaves that file, you can now control the fully open file.
Try to avoid bringing your rooks out into the board too early in the game, because like the queen they can easily be threatened by bishops and knights, which forces you to move your rook or trade it for a lesser piece. Adding to this problem is the fact that rooks are more difficult to move around a cluttered board to avoid danger.