Defense is probably one of the easiest skills to teach and one of the easiest to learn. Your players will improve in a matter of weeks. The following principles are what you should focus on in practice. Remember, practice is the time to learn not game time.
Basic Defensive Principles
Body Position - Defensive body position will enable the defender to move quickly in all directions. The feet should be slightly wider than shoulder width apart. The knees bent, butt down and the back straight. The weight should be on the balls of the feet. The feet should come off of the floor slightly and slide. You DO NOT want to hop. The jump-step is ok when attacking but you don't want to see your defenders' feet come very far of the floor. At the same time you don't want your defenders sliding as they will to slow to stay with the offensive player.
Staying with the offensive player - you need to practice your defender maintaining a distance of 1 foot to 3 yards away from the offensive position, depending on the position of the offensive player and the ball.
Force the offensive players movement - You always want to teach your defenders to force the offensive players movement even when the offensive player being guarded does not have the ball. Being away of which way the offense is moving and prevent the offensive player from moving to that area. Forcing the offensive players movement is accomplished by setting up at a 45° angle to the defense, one foot to the side. This positioning makes it easier for the offense to go around you one way. You need to practice forcing from each position around the basket.
Step up your defense around the basket - Prevent low post players from moving to where they want to go. Make sure that the low post always has a body on them when they are around the basket. This also ensures that the defender will be in position to block out for the defensive rebound.
The pick is difficult to defend - coach your defenders to move on top of the pick. Drill your players on communication. They need to let their teammates know that the pick is coming so they are prepared to fight over the top of the pick when the offensive player they are guarding had the ball.
Strong and weak side defense needs to understand their positioning - Weak side defenders need to playoff their man if they are more than one pass away from the ball they need to be prepared to help out defensively if a teammate is beaten. Picking up uncovered players moving to the basket is critical for your weak side defenders.