If the league allows rookies, then you will have access to the Inactive List. If it's available, you must have 3 players on it at all times and those players won't play in games. When you want to activate a player, you must then select an active player to inactivate.
You may move players on or off the Inactive list throughout the season, including the playoffs and there is no limit to the number of moves than you can make to and from this list.